res_pjsip_aoc: Don't assume a body exists on responses.

When adding AOC to an outgoing response the code
assumed that a body would exist for comparing the
Content-Type. This isn't always true.

The code now checks to make sure the response has
a body before checking the Content-Type.

ASTERISK-21502

Change-Id: Iaead371434fc3bc693dad487228106a7d7a5ac76

res/res_pjsip_aoc.c

@@ -628,7 +628,7 @@ static void aoc_invite_outgoing_response(struct ast_sip_session *session,
 		return;
 	}
 
-	if (pjsip_media_type_cmp(&tdata->msg->body->content_type,
+	if (tdata->msg->body && pjsip_media_type_cmp(&tdata->msg->body->content_type,
 			&pjsip_media_type_multipart_mixed, 0) == 0) {
 		multipart_body = tdata->msg->body;
 	} else {