add missing else

Summary: Closes https://github.com/facebook/rocksdb/pull/3121

Differential Revision: D6229415

Pulled By: miasantreble

fbshipit-source-id: 57c7ad2fddf5dd6b8d7e3aaf6f62348151327dfb

util/crc32c.cc

@@ -469,7 +469,9 @@ std::string IsFastCrc32Supported() {
   if (has_fast_crc) {
     fast_zero_msg.append("Supported on " + arch);
   }
-  fast_zero_msg.append("Not supported on " + arch);
+  else {
+    fast_zero_msg.append("Not supported on " + arch);
+  }
   return fast_zero_msg;
 }