regex transformation: Fix a double free bug

Supplying a bad regular expression to {re.subst} would lead to
a double pkg_free().

Credits to Nick Altmann for reporting / helping with troubleshooting

(cherry picked from commit 00d44e8)

re.c

@@ -289,7 +289,6 @@ struct subst_expr* subst_parser(str* subst)
 	c=*re_end; /* regcomp expects null terminated strings -- save */
 	*re_end=0;
 	if (regcomp(regex, re, cflags)!=0){
-		pkg_free(regex);
 		*re_end=c; /* restore */
 		LM_ERR("bad regular expression %.*s in %.*s\n",
 				(int)(re_end-re), re, subst->len, subst->s);