Return and warn if a handle's refcount is <= 0

This makes it more likely consumers notice invalid unreferences.
GObject has the same assertion.

http://bugzilla.openedhand.com/show_bug.cgi?id=2029

Signed-off-by: Emmanuele Bassi <ebassi@linux.intel.com>

cogl/cogl-util.c

@@ -78,6 +78,7 @@ cogl_handle_unref (CoglHandle handle)
   CoglHandleObject *obj = (CoglHandleObject *)handle;
 
   g_return_if_fail (handle != COGL_INVALID_HANDLE);
+  g_return_if_fail (obj->ref_count > 0);
 
   if (--obj->ref_count < 1)
     {