res_pjsip_pubsub: Increment persistence data ref when recreating.

Each subscription needs to have a reference to the persisted data
for it, as well as the main JSON contained within the tree. When
recreating a subscription this did not occur and they both shared
the same reference.

ASTERISK-28714

Change-Id: I706abd49ea182ea367a4ac3feca2706460ae9f4a