res_pjsip_pubsub: Fix ao2 reference leak of subscription tree in ast_sip_subscription

res_pjsip_pubsub: Fix ao2 reference leak of subscription tree in ast_sip_subscription

allocate_subscription() increments the ao2 reference count of the subscription tree,
but the reference was not consistently released during subscription destruction,
resulting in leaked sip_subscription_tree objects.

This patch makes destroy_subscription() responsible for releasing sub->tree,
removes ad-hoc cleanup in error paths,
and guards tree cleanup to ensure refcount symmetry and correct ownership.

Fixes: #1703