gdbusmessage: Fix a minor memory leak on an error path

If g_dbus_message_to_blob() fails at all, it will leak its mbuf. Spotted
by running the gdbus-serialization test under Valgrind — so there is a
justification for leak-free tests after all!

gio/gdbusmessage.c

@@ -2745,6 +2745,9 @@ g_dbus_message_to_blob (GDBusMessage          *message,
   ret = (guchar *)mbuf.data;
 
  out:
+  if (ret == NULL)
+    g_free (mbuf.data);
+
   return ret;
 }