nathelper: remove magic constant

(cherry picked from commit 3d187336f22e30800acefd28e4d0f98e0f677a49)
(cherry picked from commit e8dc8506dbc4d8964e02c3be175d7f18f5fe69a4)

src/modules/nathelper/nathelper.c

@@ -1153,7 +1153,7 @@ static int ki_handle_ruri_alias_mode(struct sip_msg *msg, int mode)
 	}
 	start = NULL;
 	/* locate last alias parameter */
-	while(rest_len > _ksr_contact_alias.len + 4) {
+	while(rest_len > _ksr_contact_alias.len) {
 		if(strncmp(rest, _ksr_contact_alias.s, _ksr_contact_alias.len) == 0) {
 			start = rest;
 			if(mode == 0) {