蔡渠棠 f29e6b3be2 bugfix: MemTableList::RemoveOldMemTables invalid iterator after remov… (#6013)
Summary:
Fix issue https://github.com/facebook/rocksdb/issues/6012.

I found that it may be caused by the following codes in function _RemoveOldMemTables()_ in **db/memtable_list.cc**  :
```
  for (auto it = memlist.rbegin(); it != memlist.rend(); ++it) {
    MemTable* mem = *it;
    if (mem->GetNextLogNumber() > log_number) {
      break;
    }
    current_->Remove(mem, to_delete);
```

The iterator **it** turns invalid after `current_->Remove(mem, to_delete);`
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6013

Test Plan:
```
make check
```

Differential Revision: D18401107

Pulled By: riversand963

fbshipit-source-id: bf0da3b868ed70f7aff24cf7b3e2049c0c5c7a4e
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