res_rtp_asterisk: timestamp should be unsigned instead of signed int

Using timestamp with signed int will cause timestamps exceeding max value
to be negative.
This causes the jitterbuffer to do passthrough of the packet.

ASTERISK-28421

Change-Id: I9dabd0718180f2978856c50f43aac4e52dc3cde9

res/res_rtp_asterisk.c

@@ -6728,7 +6728,8 @@ static struct ast_frame *ast_rtp_interpret(struct ast_rtp_instance *instance, st
 	unsigned int *rtpheader = (unsigned int*)(read_area);
 	struct ast_rtp *rtp = ast_rtp_instance_get_data(instance);
 	struct ast_rtp_instance *instance1;
-	int res = length, hdrlen = 12, seqno, timestamp, payloadtype, padding, mark, ext, cc;
+	int res = length, hdrlen = 12, seqno, payloadtype, padding, mark, ext, cc;
+	unsigned int timestamp;
 	RAII_VAR(struct ast_rtp_payload_type *, payload, NULL, ao2_cleanup);
 	struct frame_list frames;