glx: Add missing null check in __glX_send_client_info()

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/glx/clientinfo.c

@@ -113,6 +113,10 @@ __glX_send_client_info(struct glx_display *glx_dpy)
    }
 
    gl_extension_string = __glXGetClientGLExtensionString();
+   if (gl_extension_string == NULL) {
+      return;
+   }
+
    gl_extension_length = strlen(gl_extension_string) + 1;
 
    c = XGetXCBConnection(glx_dpy->dpy);