xlib: fix glXDestroyContext in Gallium frontends

xlib: fix glXDestroyContext in Gallium frontends

when glx is built with -Dglx=xlib, the mishandle in
glXDestroyContext causes glmark2 to exit unexpectedly.

Error: Glmark2 needs OpenGL(ES) version >= 2.0 to run (but version string is: '(null)')!
Error: Failed to add vertex shader from file None:
Error:   Failed to create the new program
[build] <default>: Set up failed

Gitlab: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3985

Signed-off-by: Luc Ma <luc@sietium.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21067>