[mutter] cogl: Do not unref a NULL object

[Jonas Ådahl <jadahl src gnome org>]

commit 3104d697c0c66748ba0b005c145061499b3f1090
Author: Marco Trevisan (Treviño) <mail 3v1n0 net>
Date:   Thu Apr 19 04:49:01 2018 -0500

    cogl: Do not unref a NULL object
    
    Add return-if-fail statement to avoid deferencing NULL object

 cogl/cogl/cogl-object.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
---
diff --git a/cogl/cogl/cogl-object.c b/cogl/cogl/cogl-object.c
index 1b849d58c..e159c6c3a 100644
--- a/cogl/cogl/cogl-object.c
+++ b/cogl/cogl/cogl-object.c
@@ -109,7 +109,11 @@ _cogl_object_default_unref (void *object)
 void
 cogl_object_unref (void *obj)
 {
-  void (* unref_func) (void *) = ((CoglObject *) obj)->klass->virt_unref;
+  void (* unref_func) (void *);
+
+  _COGL_RETURN_IF_FAIL (obj != NULL);
+
+  unref_func = ((CoglObject *) obj)->klass->virt_unref;
   unref_func (obj);
 }