gerror: Don't leak memory when overwrite warning

Even though we can't always make no-leak guarantees when g_warning()
in this case we're testing this behavior in tests, and it would be
good to be able to valgrind this.

https://bugzilla.gnome.org/show_bug.cgi?id=711751

glib/tests/error.c

@@ -28,10 +28,7 @@ test_overwrite (void)
   g_test_assert_expected_messages ();
 
   g_assert_error (dest, G_MARKUP_ERROR, G_MARKUP_ERROR_EMPTY);
-  g_assert_error (src, G_MARKUP_ERROR, G_MARKUP_ERROR_PARSE);
   g_error_free (dest);
-  g_error_free (src);
-
 }
 
 static void