res_rtp_asterisk.c: Check for first DTMF having timestamp set to 0

When the first DTMF receive in RF2833 codec have TimeStamp at 0
is not processed.

ASTERISK-28812

Change-Id: I3196803a062dd2daee4938c9a778c3810cb7e504

res/res_rtp_asterisk.c

@@ -5492,7 +5492,7 @@ static void process_dtmf_rfc2833(struct ast_rtp_instance *instance, unsigned cha
 
 		if (event_end & 0x80) {
 			/* End event */
-			if ((rtp->last_seqno != seqno) && (timestamp > rtp->last_end_timestamp)) {
+			if ((rtp->last_seqno != seqno) && ((timestamp > rtp->last_end_timestamp) || ((timestamp == 0) && (rtp->last_end_timestamp == 0)))) {
 				rtp->last_end_timestamp = timestamp;
 				rtp->dtmf_duration = new_duration;
 				rtp->resp = resp;