call to a member function delete() on null

Problem/Motivation

In certain situations, a rule can get deleted before the entity that references it is deleted. This results in an error when smart_date_recur_entity_delete() tries to delete the rule and finds it already gone.

Steps to reproduce

I don't know about a vanilla install, but on our site, this occurs when a node is deleted by its owner rather than by an administrator.

Proposed resolution

Check that the rrule object exists before trying to delete it.

Remaining tasks

Make a patch.