Prevent a crash in res_pjsip_dtmf_info.c

This change makes sure that a content type header exists before
checking the contents of the header against known SIP INFO DTMF content
types.
........

Merged revisions 398206 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@398207 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_pjsip_dtmf_info.c

@@ -34,8 +34,9 @@
 
 static int is_media_type(pjsip_rx_data *rdata, char *subtype)
 {
-	return !pj_strcmp2(&rdata->msg_info.ctype->media.type, "application") &&
-		!pj_strcmp2(&rdata->msg_info.ctype->media.subtype, subtype);
+	return rdata->msg_info.ctype
+		&& !pj_strcmp2(&rdata->msg_info.ctype->media.type, "application")
+		&& !pj_strcmp2(&rdata->msg_info.ctype->media.subtype, subtype);
 }
 
 static void send_response(struct ast_sip_session *session,