res_pjsip_pubsub: Fix crash on destruction of empty subscription tree.

If an old persistent subscription is recreated but then immediately
destroyed because it is out of date, the subscription tree will have no
leaf subscriptions on it. This was resulting in a crash when attempting
to destroy the subscription tree.

A simple NULL check fixes this problem.

Change-Id: I85570b9e2bcc7260a3fe0ad85904b2a9bf36d2ac