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
........

Merged revisions 409156 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@409157 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -22868,9 +22868,11 @@ static void handle_response_invite(struct sip_pvt *p, int resp, const char *rest
 					/* This 200 OK's SDP is not acceptable, so we need to ack, then hangup */
 					/* For re-invites, we try to recover */
 					ast_set_flag(&p->flags[0], SIP_PENDINGBYE);
-					ast_channel_hangupcause_set(p->owner, AST_CAUSE_BEARERCAPABILITY_NOTAVAIL);
 					p->hangupcause = AST_CAUSE_BEARERCAPABILITY_NOTAVAIL;
-					sip_queue_hangup_cause(p, AST_CAUSE_BEARERCAPABILITY_NOTAVAIL);
+					if (p->owner) {
+						ast_channel_hangupcause_set(p->owner, AST_CAUSE_BEARERCAPABILITY_NOTAVAIL);
+						sip_queue_hangup_cause(p, AST_CAUSE_BEARERCAPABILITY_NOTAVAIL);
+					}
 				}
 			}
 			ast_rtp_instance_activate(p->rtp);