http-push.c: make CURLOPT_IOCTLDATA a usable pointer

http-push.c: make CURLOPT_IOCTLDATA a usable pointer

Fixes a small bug affecting push to remotes which use some sort of
multi-pass authentication. In particular the bug affected SabreDAV as
configured by Box.com [1].

It must be a weird server configuration for the bug to have survived
this long. Someone should write a test for it.

[1] http://marc.info/?l=git&m=140460482604482

Signed-off-by: Abbaad Haider <abbaad@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>