glx: fix uninitialized var in __glXDRIscreenProbe

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=59825

Signed-off-by: Piotr Dziwinski <piotrdz@gmail.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>
(cherry picked from commit cc3d1a5a6120e721a46c67446ba68f5596055633)