glx: guard swap-interval functions against destroyed drawables

glx: guard swap-interval functions against destroyed drawables

The GLX specification says about glXDestroyPixmap:

    "The storage for the GLX pixmap will be freed when it is not current
     to any client."

So arguably, functions like glXSwapIntervalMESA can be called after
glXDestroyPixmap has been called for the currently bound GLXPixmap.
In that case, the GLXDRIDrawable no longer exists, and so we just skip
those calls.

Cc: 17.0 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>