MT#63317 re-fix extension parsing

Basically a revert of Ie62fa4ed

Maybe this is correct now?

Change-Id: Ia2b1b5d35c2a209430a23110f1b929e2b1ac80fb
(cherry picked from commit 8a3d8c911d)

kernel-module/xt_RTPENGINE.c

@@ -5103,7 +5103,7 @@ static void parse_rtp(struct rtp_parsed *rtp, struct sk_buff *skb) {
 		if (rtp->payload_len < sizeof(*ext))
 			goto error;
 		ext = (void *) rtp->payload;
-		ext_len = sizeof(*ext) + 4 + ntohs(ext->length) * 4;
+		ext_len = sizeof(*ext) + ntohs(ext->length) * 4;
 		if (rtp->payload_len < ext_len)
 			goto error;
 		rtp->payload += ext_len;

lib/rtplib.c

@@ -91,7 +91,7 @@ int rtp_payload(struct rtp_header **out, str *p, const str *s) {
 			goto error;
 		ext = (void *) p->s;
 		err = "short packet (header extensions)";
-		if (str_shift(p, sizeof(*ext) + 4 + ntohs(ext->length) * 4))
+		if (str_shift(p, sizeof(*ext) + ntohs(ext->length) * 4))
 			goto error;
 	}