[mutter] core/screen: Make logical monitor getters handle being headless

[Jonas Ådahl <jadahl src gnome org>]

commit 24c91d95de97a3313591703c61297da4ca7dc6d3
Author: Jonas Ådahl <jadahl gmail com>
Date:   Wed Apr 12 14:24:25 2017 +0800

    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 |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)
---
diff --git a/src/core/screen.c b/src/core/screen.c
index 503c201..98c5538 100644
--- a/src/core/screen.c
+++ b/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;
 }
 
 /**