mesa/st: release debug_output after destroying the context

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3230
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/2218
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5789>
1 file changed