libfetch: Plug leak in 304 Not Modified case

When we get a 304 Not Modified response, we need to go through the error
path to properly free any resources we've allocated instead of just
returning NULL directly.

MFC after:	1 week
Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D60016

lib/libfetch/http.c

@@ -1986,7 +1986,7 @@ http_request_body(struct url *URL, const char *op, struct url_stat *us,
 
 	if (conn->err == HTTP_NOT_MODIFIED) {
 		http_seterr(HTTP_NOT_MODIFIED);
-		return (NULL);
+		goto ouch;
 	}
 
 	/* check for inconsistencies */