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.
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Merged revisions 423894 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 423895 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@423896 65c4cc65-6c06-0410-ace0-fbb531ad65f3