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] : Set up failed
Gitlab: #3985
Signed-off-by: Luc Ma luc@sietium.com