http: clear POSTFIELDS when initializing a slot
After posting a short request using CURLOPT_POSTFIELDS, if the slot
is reused for posting a large payload, the slot ends up having both
POSTFIELDS (which now points at a random garbage) and READFUNCTION,
in which case the curl library tries to use the stale POSTFIELDS.
Clear it as part of the general slot initialization in get_active_slot().
Heavylifting-by: Shawn Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Shawn Pearce <spearce@spearce.org>