http.c: set slot callback members to NULL when releasing object

http.c: set slot callback members to NULL when releasing object

Set the members callback_func and callback_data of freq->slot to NULL
when releasing a http_object_request. release_active_slot() is also
invoked on the slot to remove the curl handle associated with the slot
from the multi stack (CURLM *curlm in http.c).

These prevent the callback function and data from being used in http
methods (like http.c::finish_active_slot()) after a
http_object_request has been free'd.

Noticed by Ali Polatel, who later tested this patch to verify that it
fixes the problem he saw; Dscho helped to identify the problem spot.

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