Fix Windows build broken by 5c97a7c066

Summary:
A typo conversion fails Windows build. Fix it.
Closes https://github.com/facebook/rocksdb/pull/2500

Differential Revision: D5325962

Pulled By: siying

fbshipit-source-id: 2cefdafc9afbc85f856f403af7c876b622400630

db/db_io_failure_test.cc

@@ -283,7 +283,8 @@ TEST_F(DBIOFailureTest, FlushSstRangeSyncError) {
   rocksdb::SyncPoint::GetInstance()->EnableProcessing();
 
   Random rnd(301);
-  std::string rnd_str = RandomString(&rnd, options.bytes_per_sync / 2);
+  std::string rnd_str =
+      RandomString(&rnd, static_cast<int>(options.bytes_per_sync / 2));
   std::string rnd_str_512kb = RandomString(&rnd, 512 * 1024);
 
   ASSERT_OK(Put(1, "foo", "bar"));