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

db/memtable_list.cc

@@ -736,17 +736,24 @@ void MemTableList::RemoveOldMemTables(uint64_t log_number,
   assert(to_delete != nullptr);
   InstallNewVersion();
   auto& memlist = current_->memlist_;
+  autovector<MemTable*> old_memtables;
   for (auto it = memlist.rbegin(); it != memlist.rend(); ++it) {
     MemTable* mem = *it;
     if (mem->GetNextLogNumber() > log_number) {
       break;
     }
+    old_memtables.push_back(mem);
+  }
+
+  for (auto it = old_memtables.begin(); it != old_memtables.end(); ++it) {
+    MemTable* mem = *it;
     current_->Remove(mem, to_delete);
     --num_flush_not_started_;
     if (0 == num_flush_not_started_) {
       imm_flush_needed.store(false, std::memory_order_release);
     }
   }
+
   UpdateMemoryUsageExcludingLast();
   ResetTrimHistoryNeeded();
 }