Fix uninitialized prev_block_offset_ in BlockBasedTableReader (#5507)

Summary:
Found by valgrind_check.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5507

Differential Revision: D16002612

Pulled By: miasantreble

fbshipit-source-id: 13c11c183190e0a0571844635457d434da3ac59a

table/block_based/block_based_table_reader.h

@@ -755,7 +755,7 @@ class BlockBasedTableIterator : public InternalIteratorBase<TValue> {
   bool need_upper_bound_check_;
   const SliceTransform* prefix_extractor_;
   BlockType block_type_;
-  uint64_t prev_block_offset_;
+  uint64_t prev_block_offset_ = std::numeric_limits<uint64_t>::max();
   BlockCacheLookupContext lookup_context_;
   // Readahead size used in compaction, its value is used only if
   // lookup_context_.caller = kCompaction.