init filters_in_partition_

Summary:
Valgrind reports that it is not initialized.
Closes https://github.com/facebook/rocksdb/pull/2541

Differential Revision: D5376084

Pulled By: maysamyabandeh

fbshipit-source-id: 55c312f4f506863aa0d25ff92c8c34b57f48b860

table/partitioned_filter_block.cc

@@ -25,7 +25,8 @@ PartitionedFilterBlockBuilder::PartitionedFilterBlockBuilder(
     : FullFilterBlockBuilder(prefix_extractor, whole_key_filtering,
                              filter_bits_builder),
       index_on_filter_block_builder_(index_block_restart_interval),
-      p_index_builder_(p_index_builder) {
+      p_index_builder_(p_index_builder),
+      filters_in_partition_(0) {
   filters_per_partition_ =
       filter_bits_builder_->CalculateNumEntry(partition_size);
 }