res_pjsip_pubsub: Ensure dialog manipulation happens on proper thread.

When destroying a subscription we remove the serializer from its dialog
and decrease its reference count. Depending on which thread dropped the
subscription reference count to 0 it was possible for this to occur in
a thread where it is not possible.

(closes issue ASTERISK-22952)
Reported by: Matt Jordan


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@404553 f38db490-d61c-443f-a65b-d21fe96a405b