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 <email@example.com> Signed-off-by: Junio C Hamano <firstname.lastname@example.org>