weather group: descriptor: -FZRA BC causes exception

PAEH 290431Z AUTO 11002KT 10SM -FZRA BC BKN002 M08/M08 A3010 RMK AO2 SLP208

not sure if that is correct fix...
```
                // if we have phenomena (we should always!)
                String phenomena = ((String) tokens.get(index));
                if (pos < phenomena.length() && (
                        ((String) tokens.get(index)).substring(pos, pos + 2)
```