x11.h: fix querying monitors that are connected but turned off

This also fixes the following issues under the same conditions:
- `get-cursor-monitor`: crashes
- `blank-screens`: enters an infinite loop and uses up all memory
  until OOM kicks in

src/c/include/x11.h

@@ -41,7 +41,7 @@ char *get_cursor_monitor(Display *display)
 
     for (i = 0; i < screen_resources->noutput; i++) {
         output_info = XRRGetOutputInfo(display, screen_resources, screen_resources->outputs[i]);
-        if (!output_info || output_info->connection != RR_Connected) {
+        if (!output_info || output_info->connection != RR_Connected || output_info->crtc == None) {
             if (output_info)
                 XRRFreeOutputInfo(output_info);
             continue;