xlib: check for null ctx pointer in glXIsDirect()

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=64745
Note: This is a candidate for the stable branches.

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

src/mesa/drivers/x11/fakeglx.c

@@ -1551,7 +1551,7 @@ Fake_glXIsDirect( Display *dpy, GLXContext ctx )
 {
    struct fake_glx_context *glxCtx = (struct fake_glx_context *) ctx;
    (void) dpy;
-   return glxCtx->xmesaContext->direct;
+   return glxCtx ? glxCtx->xmesaContext->direct : False;
 }