res_pjsip.c: Add missing off nominal cleanup in ast_sip_push_task_synchronous().

* Made memset the std struct in ast_sip_push_task_synchronous() because if
DEBUG_THREADS is enabled then uninitialized lock tracking data is used.
........

Merged revisions 423894 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@423895 f38db490-d61c-443f-a65b-d21fe96a405b