http-push: check path length before using it

We use path_len to skip the base url/path, but we do not know for sure
if path does indeed contain the base url/path. Check if this is so.

Helped-by: Johnathan Nieder <jrnieder@gmail.com>
Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>