st/mesa: fix glReadBuffer(GL_NONE) segfault
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=73956 Cc: 10.0 mesa-stable@lists.freedesktop.org Tested-by: Ahmed Allam ahmabdabd@hotmail.com Reviewed-by: Marek Olšák marek.olsak@amd.com (cherry picked from commit f7c118ff)