glx: Fix segfault when Nvidia PRIME render offload is enabled, but not used

Fixes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/11719

Acked-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37342>

src/glx/clientinfo.c

@@ -123,6 +123,10 @@ glxSendClientInfo(struct glx_display *glx_dpy, int screen)
    for (i = 0; i < ScreenCount(glx_dpy->dpy); i++) {
       struct glx_screen *src = glx_dpy->screens[i];
 
+      if (src == NULL) {
+          continue;
+      }
+
       const char *haystack = src->serverGLXexts;
       while (haystack != NULL) {
     char *match = strstr(haystack, "GLX_ARB_create_context");