fix coredump for release nullptr

Summary:
Coredump will be triggered when ingest external sst file after delete range.
ref https://github.com/facebook/rocksdb/issues/2398
Closes https://github.com/facebook/rocksdb/pull/2463

Differential Revision: D5275599

Pulled By: ajkr

fbshipit-source-id: 0828dbc062ea8c74e913877cd63494fd3478a30d

db/table_cache.cc

@@ -286,9 +286,11 @@ InternalIterator* TableCache::NewRangeTombstoneIterator(
   if (s.ok()) {
     result = table_reader->NewRangeTombstoneIterator(options);
     if (result != nullptr) {
+      if (handle != nullptr) {
         result->RegisterCleanup(&UnrefEntry, cache_, handle);
       }
     }
+  }
   if (result == nullptr && handle != nullptr) {
     // the range deletion block didn't exist, or there was a failure between
     // getting handle and getting iterator.