Fix a TSAN failure (#4250)

Summary:
TSAN fails due to comparison between signed int and unsigned long. Fix it by
static_casting.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4250

Differential Revision: D9256535

Pulled By: riversand963

fbshipit-source-id: c6bad23ff70c6d0ec58e2e85c401ce0ad45de609

tools/sst_dump_test.cc

@@ -110,7 +110,7 @@ public:
   template<std::size_t N>
   void PopulateCommandArgs(const std::string& file_path, const char* command,
     char* (&usage)[N]) const {
-    for (int i = 0; i < N; ++i) {
+    for (int i = 0; i < static_cast<int>(N); ++i) {
       usage[i] = new char[optLength];
     }
     snprintf(usage[0], optLength, "./sst_dump");