http.c: avoid freeing an uninitialized pointer

An earlier 59b8d38f (http.c: remove verification of remote packs) left
the variable "url" uninitialized; "goto cleanup" codepath can free it
which is not very nice.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

http.c

@@ -866,7 +866,7 @@ static int fetch_pack_index(unsigned char *sha1, const char *base_url)
 	int ret = 0;
 	char *hex = xstrdup(sha1_to_hex(sha1));
 	char *filename;
-	char *url;
+	char *url = NULL;
 	struct strbuf buf = STRBUF_INIT;
 
 	if (has_pack_index(sha1)) {