nathelper (k): fix for flag 'l'

- fix SF: #2814137 - handle re-invite with force_rtp_proxy("l");
- patch by Marcus Hunger

modules_k/nathelper/nathelper.c

@@ -2608,7 +2608,7 @@ force_rtp_proxy(struct sip_msg* msg, char* str1, char* str2, int offer)
 		if (to_tag.len == 0)
 			return -1;
 		create = 0;
-		if (swap != 0) {
+		if (swap != 0 || (msg->first_line.type == SIP_REPLY && offer != 0)) {
 			tmp = from_tag;
 			from_tag = to_tag;
 			to_tag = tmp;