chan_sip: Add precautionary p->owner checks.

* Add precautionary p->owner checks in sip_hangup(), get_refer_info(),
get_also_info(), and interpret_t38_parameters().

* Simplify some tangled logic in get_refer_info(), get_also_info(), and
add_rpid().

* Removed some dead code in handle_request_invite().

(closes issue ASTERISK-23323)
Reported by: Walter Doekes
Patches:
      issueA23323-more_p_owner_checks-1.8.x.patch (license #5674) uploaded by wdoekes (modified)
      issueA23323-more_p_owner_checks-11.x.patch (license #5674) uploaded by wdoekes (modified)
      issueA23323-more_p_owner_checks-12.x.patch (license #5674) uploaded by wdoekes (modified)
      issueA23323-more_p_owner_checks-trunk.patch (license #5674) uploaded by wdoekes (modified)
........

Merged revisions 409207 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@409255 65c4cc65-6c06-0410-ace0-fbb531ad65f3