res/res_pjsip_pubsub: Gracefully handle errors when re-creating subscriptions

A subscription that has been persisted can - for various reasons - fail to be
re-created on startup. This patch resolves a number of crashes that occurred
when a subscription cannot be re-created on several off-nominal paths.

#SIPit31

ASTERISK-24368 #close
Reported by: Matt Jordan


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@424601 f38db490-d61c-443f-a65b-d21fe96a405b