relative_url(): fix incorrect condition

In 63e95beb08 (submodule: port resolve_relative_url from shell to C,
2016-04-15), we added a loop over `url` where we are looking for `../`
or `./` components.

The loop condition we used is the pointer `url` itself, which is clearly
not what we wanted.

Pointed out by Coverity.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

remote.c

@@ -2846,7 +2846,7 @@ char *relative_url(const char *remote_url, const char *url,
 	 * When the url starts with '../', remove that and the
 	 * last directory in remoteurl.
 	 */
-	while (url) {
+	while (*url) {
 		if (starts_with_dot_dot_slash_native(url)) {
 			url += 3;
 			colonsep |= chop_last_dir(&remoteurl, is_relative);