res_rtp_asterisk.c: Set Mark on rtp when timestamp skew is too big

Set Mark bit in rtp stream when timestamp skew is bigger than MAX_TIMESTAMP_SKEW.

Fixes: #927
(cherry picked from commit 2599aa3be0)

res/res_rtp_asterisk.c

@@ -5267,6 +5267,11 @@ static int rtp_raw_write(struct ast_rtp_instance *instance, struct ast_frame *fr
 	}
 
 	if (ast_test_flag(frame, AST_FRFLAG_HAS_TIMING_INFO)) {
+		if (abs(frame->ts * rate - (int)rtp->lastts) > MAX_TIMESTAMP_SKEW) {
+			ast_verbose("(%p) RTP audio difference is %d set mark\n",
+				instance, abs(frame->ts * rate - (int)rtp->lastts));
+			mark = 1;
+		}
 		rtp->lastts = frame->ts * rate;
 	}