Fix MSVC compiler warning.

src/gdef.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "gdef.h"
 
 #include <vector>
 
 #include "layout.h"
 #include "maxp.h"
@@ skipping 332 matching lines @@
       DROP_THIS_TABLE;
       return true;
     }
   }
   gdef->data = data;
   gdef->length = length;
   return true;
 }
 
 bool ots_gdef_should_serialise(OpenTypeFile *file) {
-  return file->gdef;
+  return file->gdef != NULL;
 }
 
 bool ots_gdef_serialise(OTSStream *out, OpenTypeFile *file) {
   if (!out->Write(file->gdef->data, file->gdef->length)) {
     return OTS_FAILURE();
   }
 
   return true;
 }
 
 void ots_gdef_free(OpenTypeFile *file) {
   delete file->gdef;
 }
 
 }  // namespace ots