Xext: fix typo in condition.

Fall-out from dc3aba8a55.
We must free the event if it is NOT an extension event.

Xext/xtest.c

@@ -500,7 +500,7 @@ ProcXTestFakeInput(client)
         xfree(master_event);
     } else
         (*dev->public.processInputProc)(ev, dev, nev);
-    if (extension)
+    if (!extension)
         xfree(ev);
     return client->noClientException;
 }