rocksdb: member function call and member invalidation

[qqiangwu]

void ColumnFamilyData::CreateNewMemtable(
    const MutableCFOptions& mutable_cf_options, SequenceNumber earliest_seq) {
  if (mem_ != nullptr) {
    delete mem_->Unref();
  }
  SetMemtable(ConstructNewMemtable(mutable_cf_options, earliest_seq));
  mem_->Ref();
}

gives:

➜  rocksdb-main git:(main) ✗ cppsafe -p build db/column_family.cc
/Users/wuqq/dev/rocksdb-main/db/column_family.cc:1137:3: warning: passing a dangling pointer as argument
 1137 |   mem_->Ref();
      |   ^~~~
/Users/wuqq/dev/rocksdb-main/db/column_family.cc:1134:5: note: deleted here
 1134 |     delete mem_->Unref();
      |     ^~~~~~~~~~~~~~~~~~~~
1 warning generated.