http: clear POSTFIELDS when initializing a slot

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>
Acked-by: Shawn Pearce <redacted>