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>

http.c

@@ -494,6 +494,7 @@ struct active_request_slot *get_active_slot(void)
 	curl_easy_setopt(slot->curl, CURLOPT_CUSTOMREQUEST, NULL);
 	curl_easy_setopt(slot->curl, CURLOPT_READFUNCTION, NULL);
 	curl_easy_setopt(slot->curl, CURLOPT_WRITEFUNCTION, NULL);
+	curl_easy_setopt(slot->curl, CURLOPT_POSTFIELDS, NULL);
 	curl_easy_setopt(slot->curl, CURLOPT_UPLOAD, 0);
 	curl_easy_setopt(slot->curl, CURLOPT_HTTPGET, 1);