libagx: add missing null pointer check

fixes KHR-GL46.pipeline_statistics_query_tests_ARB.functional_tess_queries
with hard fault

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33682>
1 file changed