core/screen: Make logical monitor getters handle being headless

Don't crash or return invalid indices when we are headless.

https://bugzilla.gnome.org/show_bug.cgi?id=730551

src/core/screen.c

@@ -1515,6 +1515,9 @@ meta_screen_get_monitor_index_for_rect (MetaScreen    *screen,
 
   logical_monitor =
     meta_monitor_manager_get_logical_monitor_from_rect (monitor_manager, rect);
+  if (!logical_monitor)
+    return -1;
+
   return logical_monitor->number;
 }
 
@@ -1604,7 +1607,7 @@ meta_screen_get_primary_monitor (MetaScreen *screen)
   if (logical_monitor)
     return logical_monitor->number;
   else
-    return 0;
+    return -1;
 }
 
 /**