http*: cleanup slot->local after fclose

http*: cleanup slot->local after fclose

Set slot->local to NULL after doing a fclose() on the file it points
to. This prevents the passing of a FILE* pointer to a fclose()'d file
to ftell() in http.c::run_active_slot().

This issue was raised by Clemens Buchacher on 30th May 2009:

  http://www.spinics.net/lists/git/msg104623.html

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