prevent nullptr dereference in table reader error case

Summary:
A user encountered segfault on the call to `CacheDependencies()`, probably because `NewIndexIterator()` failed before populating `*index_entry`. Let's avoid the call in that case.
Closes https://github.com/facebook/rocksdb/pull/2939

Differential Revision: D5928611

Pulled By: ajkr

fbshipit-source-id: 484be453dbb00e5e160e9c6a1bc933df7d80f574

table/block_based_table_reader.cc

@@ -813,15 +813,19 @@ Status BlockBasedTable::Open(const ImmutableCFOptions& ioptions,
       CachableEntry<IndexReader> index_entry;
       unique_ptr<InternalIterator> iter(
           new_table->NewIndexIterator(ReadOptions(), nullptr, &index_entry));
+      s = iter->status();
+      if (s.ok()) {
+        // This is the first call to NewIndexIterator() since we're in Open().
+        // On success it should give us ownership of the `CachableEntry` by
+        // populating `index_entry`.
+        assert(index_entry.value != nullptr);
         index_entry.value->CacheDependencies(pin);
         if (pin) {
           rep->index_entry = std::move(index_entry);
         } else {
           index_entry.Release(table_options.block_cache.get());
         }
-      s = iter->status();
 
-      if (s.ok()) {
         // Hack: Call GetFilter() to implicitly add filter to the block_cache
         auto filter_entry = new_table->GetFilter();
         if (filter_entry.value != nullptr) {