fix clang analyzer warnings (#4072)

Summary:
clang analyze is giving the following warnings:
> db/compaction_job.cc:1178:16: warning: Called C++ object pointer is null
    } else if (meta->smallest.size() > 0) {
               ^~~~~~~~~~~~~~~~~~~~~
db/compaction_job.cc:1201:33: warning: Access to field 'marked_for_compaction' results in a dereference of a null pointer (loaded from variable 'meta')
    meta->marked_for_compaction = sub_compact->builder->NeedCompact();
    ~~~~
db/version_set.cc:2770:26: warning: Called C++ object pointer is null
        uint32_t cf_id = last_writer->cfd->GetID();
                         ^~~~~~~~~~~~~~~~~~~~~~~~~
Closes https://github.com/facebook/rocksdb/pull/4072

Differential Revision: D8685852

Pulled By: miasantreble

fbshipit-source-id: b0e2fd9dfc1cbba2317723e09886384b9b1c9085

db/compaction_job.cc

@@ -1167,6 +1167,7 @@ Status CompactionJob::FinishCompactionOutputFile(
   // Check for iterator errors
   Status s = input_status;
   auto meta = &sub_compact->current_output()->meta;
+  assert(meta != nullptr);
   if (s.ok()) {
     Slice lower_bound_guard, upper_bound_guard;
     std::string smallest_user_key;

db/version_set.cc

@@ -2759,6 +2759,7 @@ Status VersionSet::ProcessManifestWrites(
       }
       last_writer = *(it++);
       assert(last_writer != nullptr);
+      assert(last_writer->cfd != nullptr);
       if (last_writer->cfd != nullptr && last_writer->cfd->IsDropped()) {
         continue;
       }