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>

http.c

@@ -1285,5 +1285,10 @@ void release_http_object_request(struct http_object_request *freq)
 		free(freq->url);
 		freq->url = NULL;
 	}
-	freq->slot = NULL;
+	if (freq->slot != NULL) {
+		freq->slot->callback_func = NULL;
+		freq->slot->callback_data = NULL;
+		release_active_slot(freq->slot);
+		freq->slot = NULL;
+	}
 }