rocksdb: don't call LZ4_loadDictHC with null dictionary

Summary: UBSAN revealed a pointer underflow when `LZ4HC_init_internal` is called with a null `start`.

Reviewed By: ajkr

Differential Revision: D30181874

fbshipit-source-id: ca9bbac1a85c58782871d7f153af733b000cc66c

util/compression.h

@@ -1224,8 +1224,10 @@ inline bool LZ4HC_Compress(const CompressionInfo& info,
   const char* compression_dict_data =
       compression_dict.size() > 0 ? compression_dict.data() : nullptr;
   size_t compression_dict_size = compression_dict.size();
+  if (compression_dict_data != nullptr) {
     LZ4_loadDictHC(stream, compression_dict_data,
                   static_cast<int>(compression_dict_size));
+  }
 
 #if LZ4_VERSION_NUMBER >= 10700  // r129+
   outlen =