xlib: add null ctx check in glXDestroyContext()

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=64934
NOTE: This is a candidate for the stable branches.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

src/mesa/drivers/x11/fakeglx.c

@@ -1533,6 +1533,7 @@ void _kw_ungrab_all( Display *dpy )
 static void
 Fake_glXDestroyContext( Display *dpy, GLXContext ctx )
 {
+   if (ctx) {
       struct fake_glx_context *glxCtx = (struct fake_glx_context *) ctx;
       (void) dpy;
       MakeCurrent_PrevContext = 0;
@@ -1544,6 +1545,7 @@ Fake_glXDestroyContext( Display *dpy, GLXContext ctx )
       XMesaGarbageCollect(dpy);
       free(glxCtx);
    }
+}
 
 
 static Bool