Fix refill logic in nextDoc().

[jpountz]

The recent optimization from #14164 interfered in a bad way with a prior optimization.

[jpountz]

Benchmarks suggest that the additional overhead in nextDoc() is fine:

                            TaskQPS baseline      StdDevQPS my_modified_version      StdDev                Pct diff p-value
                      OrHighRare      517.98      (4.5%)      506.55      (6.5%)   -2.2% ( -12% -    9%) 0.211
                DismaxOrHighHigh      163.10      (2.2%)      160.59      (3.4%)   -1.5% (  -6% -    4%) 0.090
             FilteredOrStopWords       71.58      (6.1%)       70.50      (6.1%)   -1.5% ( -12% -   11%) 0.433
                       OrHighMed      292.53      (2.3%)      288.61      (2.4%)   -1.3% (  -5% -    3%) 0.066
              FilteredOrHighHigh       97.76      (6.8%)       96.47      (6.0%)   -1.3% ( -13% -   12%) 0.514
                     OrStopWords       44.42      (3.4%)       43.84      (3.7%)   -1.3% (  -8% -    5%) 0.240
                      OrHighHigh       67.46      (2.2%)       66.59      (1.9%)   -1.3% (  -5% -    2%) 0.050
                        Wildcard      131.06      (3.0%)      129.44      (4.2%)   -1.2% (  -8% -    6%) 0.280
             And2Terms2StopWords      267.40      (3.7%)      264.15      (3.2%)   -1.2% (  -7% -    5%) 0.264
                    AndStopWords       40.26      (3.9%)       39.78      (3.8%)   -1.2% (  -8% -    6%) 0.322
                        PKLookup      374.88      (3.7%)      370.62      (3.9%)   -1.1% (  -8% -    6%) 0.342
                       And3Terms      236.13      (3.1%)      233.61      (3.3%)   -1.1% (  -7% -    5%) 0.287
                 DismaxOrHighMed      249.56      (2.0%)      247.08      (2.6%)   -1.0% (  -5% -    3%) 0.178
                          OrMany       25.96      (5.5%)       25.71      (4.0%)   -1.0% (  -9% -    9%) 0.529
                         Prefix3      235.88      (3.1%)      233.63      (4.0%)   -1.0% (  -7% -    6%) 0.395
              Or2Terms2StopWords      253.41      (3.5%)      251.17      (3.7%)   -0.9% (  -7% -    6%) 0.439
                  FilteredOrMany       20.15      (9.8%)       19.98      (9.3%)   -0.8% ( -18% -   20%) 0.779
                      AndHighMed      176.43      (2.6%)      174.96      (2.8%)   -0.8% (  -6% -    4%) 0.328
                     AndHighHigh       57.64      (3.0%)       57.16      (3.5%)   -0.8% (  -7% -    5%) 0.423
                FilteredOr3Terms      234.70      (6.7%)      233.02      (5.9%)   -0.7% ( -12% -   12%) 0.719
      FilteredOr2Terms2StopWords      237.64      (5.6%)      235.96      (5.3%)   -0.7% ( -11% -   10%) 0.683
                      TermDTSort      606.25      (4.7%)      602.01      (3.8%)   -0.7% (  -8% -    8%) 0.605
              CombinedOrHighHigh       29.13      (2.7%)       28.93      (2.7%)   -0.7% (  -5% -    4%) 0.432
               FilteredOrHighMed      236.89      (6.5%)      235.59      (5.9%)   -0.5% ( -12% -   12%) 0.781
               CombinedOrHighMed      116.23      (2.4%)      115.60      (2.7%)   -0.5% (  -5% -    4%) 0.503
                      DismaxTerm      823.43      (4.1%)      819.10      (5.2%)   -0.5% (  -9% -    9%) 0.721
                        Or3Terms      220.05      (3.9%)      219.03      (3.9%)   -0.5% (  -7% -    7%) 0.706
                 CountAndHighMed      425.17      (3.3%)      423.34      (2.8%)   -0.4% (  -6% -    5%) 0.656
                  TermBGroup1M1P       53.55      (2.7%)       53.35      (2.8%)   -0.4% (  -5% -    5%) 0.673
     FilteredAnd2Terms2StopWords      308.51      (2.3%)      307.49      (2.6%)   -0.3% (  -5% -    4%) 0.669
              FilteredAndHighMed      184.14      (2.4%)      183.64      (2.7%)   -0.3% (  -5% -    4%) 0.735
                AndMedOrHighHigh       91.28      (3.5%)       91.04      (3.1%)   -0.3% (  -6% -    6%) 0.803
                    CombinedTerm       50.34      (2.4%)       50.27      (3.1%)   -0.2% (  -5% -    5%) 0.863
             FilteredAndHighHigh       98.97      (4.6%)       98.83      (4.4%)   -0.1% (  -8% -    9%) 0.918
                     CountPhrase        7.07      (2.9%)        7.06      (2.7%)   -0.1% (  -5% -    5%) 0.878
                    TermGroup10K       30.48      (2.4%)       30.44      (1.8%)   -0.1% (  -4% -    4%) 0.851
               TermDayOfYearSort     3408.11      (1.9%)     3403.87      (1.5%)   -0.1% (  -3% -    3%) 0.821
                         Respell       76.33      (2.3%)       76.24      (2.4%)   -0.1% (  -4% -    4%) 0.875
                  CountOrHighMed      522.17      (3.3%)      521.60      (2.9%)   -0.1% (  -6% -    6%) 0.912
                     TermGroup1M       32.16      (2.1%)       32.14      (1.7%)   -0.1% (  -3% -    3%) 0.917
         CountFilteredOrHighHigh      221.09      (3.1%)      220.96      (2.7%)   -0.1% (  -5% -    5%) 0.949
          CountFilteredOrHighMed      241.72      (2.3%)      241.63      (1.8%)   -0.0% (  -4% -    4%) 0.953
                    TermBGroup1M       43.00      (2.6%)       42.98      (2.5%)   -0.0% (  -5% -    5%) 0.968
             CountFilteredOrMany       40.87      (4.9%)       40.86      (4.2%)   -0.0% (  -8% -    9%) 0.988
             CombinedAndHighHigh       24.90      (3.4%)       24.90      (3.1%)   -0.0% (  -6% -    6%) 0.988
                       CountTerm    15466.46      (3.6%)    15465.67      (4.6%)   -0.0% (  -7% -    8%) 0.997
                    TermGroup100       33.90      (2.4%)       33.91      (2.9%)    0.0% (  -5% -    5%) 0.993
              CombinedAndHighMed       96.43      (3.3%)       96.44      (2.9%)    0.0% (  -6% -    6%) 0.992
                 CountOrHighHigh      465.82      (6.8%)      466.21      (5.3%)    0.1% ( -11% -   13%) 0.965
                CountAndHighHigh      509.45      (6.6%)      509.90      (5.7%)    0.1% ( -11% -   13%) 0.964
                          Fuzzy1      111.93      (2.9%)      112.05      (2.6%)    0.1% (  -5% -    5%) 0.907
                          Fuzzy2      105.56      (2.6%)      105.69      (2.5%)    0.1% (  -4% -    5%) 0.887
               FilteredAnd3Terms      270.45      (2.4%)      270.82      (3.6%)    0.1% (  -5% -    6%) 0.888
                   TermTitleSort      236.14      (2.2%)      236.48      (2.2%)    0.1% (  -4% -    4%) 0.836
                   TermMonthSort     3512.78      (2.0%)     3518.37      (2.5%)    0.2% (  -4% -    4%) 0.820
            FilteredAndStopWords       81.77      (4.4%)       81.90      (4.4%)    0.2% (  -8% -    9%) 0.908
                          IntNRQ      222.55     (12.7%)      223.07      (9.2%)    0.2% ( -19% -   25%) 0.946
                    FilteredTerm      269.47      (2.4%)      270.24      (2.2%)    0.3% (  -4% -    4%) 0.693
                  FilteredPhrase       55.62      (2.7%)       55.89      (2.8%)    0.5% (  -4% -    6%) 0.575
                 AndHighOrMedMed       71.48      (2.5%)       71.83      (2.5%)    0.5% (  -4% -    5%) 0.533
             CountFilteredPhrase       50.57      (3.3%)       50.82      (2.9%)    0.5% (  -5% -    6%) 0.618
                          Phrase       23.97      (3.5%)       24.13      (3.4%)    0.6% (  -6% -    7%) 0.561
                            Term      786.42      (4.5%)      799.69      (3.5%)    1.7% (  -6% -   10%) 0.187