util: Fixes invalid assumption that return non null by function util_format_fetch_rgba_func
Fixes: e342081c ("util/format: Assert that formats are valid") Closes #7020 Signed-off-by:Yonggang Luo <luoyonggang@gmail.com> Reviewed-by:
Konstantin Seurer <konstantin.seurer@gmail.com> Part-of: <mesa/mesa!18024> (cherry picked from commit 075b72ea)