chan_sip: Fix crash in ast_channel_hangupcause_set().

* Fix crash in ast_channel_hangupcause_set() because p->owner not checked
before calling.  Regression introduced by the fix for ASTERISK-22621.

(closes issue ASTERISK-23135)
Reported by: OK

(issue ASTERISK-23323)
Reported by: Walter Doekes


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@409156 f38db490-d61c-443f-a65b-d21fe96a405b