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.