[mutter] Fix crash when cursor theme lookup failed

[Florian Müllner <fmuellner src gnome org>]

commit d9a2443e530b82d742c401206e371302a77e5338
Author: Florian Müllner <fmuellner gnome org>
Date:   Tue Apr 29 22:14:34 2014 +0200

    Fix crash when cursor theme lookup failed
    
    meta_cursor_reference_from_theme() may return %NULL on failure,
    in which case we currently crash when trying to release it.

 src/core/display.c |    3 ++-
 src/core/screen.c  |    3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/src/core/display.c b/src/core/display.c
index 041393b..147cb82 100644
--- a/src/core/display.c
+++ b/src/core/display.c
@@ -1709,7 +1709,8 @@ meta_display_set_grab_op_cursor (MetaDisplay *display,
 
   cursor_ref = meta_cursor_reference_from_theme (cursor);
   meta_cursor_tracker_set_grab_cursor (tracker, cursor_ref);
-  meta_cursor_reference_unref (cursor_ref);
+  if (cursor_ref)
+    meta_cursor_reference_unref (cursor_ref);
 
   if (meta_backend_grab_device (backend, META_VIRTUAL_CORE_POINTER_ID, timestamp))
     display->grab_have_pointer = TRUE;
diff --git a/src/core/screen.c b/src/core/screen.c
index e1dc6cd..6ceab47 100644
--- a/src/core/screen.c
+++ b/src/core/screen.c
@@ -1376,7 +1376,8 @@ meta_screen_update_cursor (MetaScreen *screen)
 
   cursor_ref = meta_cursor_reference_from_theme (cursor);
   meta_cursor_tracker_set_root_cursor (tracker, cursor_ref);
-  meta_cursor_reference_unref (cursor_ref);
+  if (cursor_ref)
+    meta_cursor_reference_unref (cursor_ref);
 
   /* Set a cursor for X11 applications that don't specify their own */
   xcursor = meta_display_create_x_cursor (display, cursor);