restore check on removed() call, quick fix for crash when removing a resource
fixes #257 (closed)
mrp_object has its ->removed() method NULLed while the underlying function mrp_object_removed(MrpObject *object) is manually called. The change restores the check on ->removed() in map_resource.c like it is done in mrp_task.c
TODO : decide what is the right way to call removed() on mrp object and change the calls accordingly for more homogeneity