core: Fix meta_screen_get_current_monitor

fixes 4595209346

We're supposed to return an index from here now, no longer a pointer
to the current monitor.

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>

src/core/screen.c

@@ -2140,7 +2140,7 @@ int
 meta_screen_get_current_monitor (MetaScreen *screen)
 {
   if (screen->n_monitor_infos == 1)
-    return &screen->monitor_infos[0];
+    return 0;
   
   /* Sadly, we have to do it this way. Yuck.
    */