glx: avoid null-deref

psc can be NULL here, so let's avoid dereferencing it.

Fixes: 34dea2b38e ("glx: unify extension binding")
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32144>

src/glx/glxext.c

@@ -923,7 +923,7 @@ AllocAndFetchScreenConfigs(Display * dpy, struct glx_display * priv, enum glx_dr
 
       if(indirect) /* Load extensions required only for indirect glx */
          glxSendClientInfo(priv, i);
-      else if (priv->driver != GLX_DRIVER_WINDOWS)
+      else if (psc && priv->driver != GLX_DRIVER_WINDOWS)
          bind_extensions(psc, psc->driverName);
    }
    if (zink && !screen_count)