glx/dri3: guard in_current_context against a disappeared drawable

Cc: 17.0 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 21ec35566be2c1aca07083a67f462618ae15fa86)
1 file changed