Bug 784889 - Use nullptr in a function that returns a pointer. r=ms2ger.

dom/base/nsGlobalWindow.cpp

@@ -6130,7 +6130,7 @@ PostMessageReadStructuredClone(JSContext* cx,
     return runtimeCallbacks->read(cx, reader, tag, data, nullptr);
   }
 
-  return JS_FALSE;
+  return nullptr;
 }
 
 static JSBool