Fix BloomFilterPolicy changes for unsigned char (ARM) (#6024)

Summary:
Bug in PR https://github.com/facebook/rocksdb/issues/5941 when char is unsigned that should only affect
assertion on unused/invalid filter metadata.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6024

Test Plan: on ARM: ./bloom_test && ./db_bloom_filter_test && ./block_based_filter_block_test && ./full_filter_block_test && ./partitioned_filter_block_test

Differential Revision: D18461206

Pulled By: pdillinger

fbshipit-source-id: 68a7c813a0b5791c05265edc03cdf52c78880e9a

table/block_based/filter_policy.cc

@@ -338,7 +338,8 @@ FilterBitsReader* BloomFilterPolicy::GetFilterBitsReader(
     return new AlwaysFalseFilter();
   }
 
-  char raw_num_probes = contents.data()[len_with_meta - 5];
+  int8_t raw_num_probes =
+      static_cast<int8_t>(contents.data()[len_with_meta - 5]);
   // NB: *num_probes > 30 and < 128 probably have not been used, because of
   // BloomFilterPolicy::initialize, unless directly calling
   // FullFilterBitsBuilder as an API, but we are leaving those cases in