http.c: Spell the null pointer as NULL

Commit 1bbcc224 ("http: refactor options to http_get_*", 28-09-2013)
changed the type of final 'options' argument of the http_get_file()
function from an int to an 'struct http_get_options' pointer.
However, it neglected to update the (single) call site. Since this
call was passing '0' to that argument, it was (correctly) being
interpreted as a null pointer. Change to argument to NULL.

Noticed by sparse. ("Using plain integer as NULL pointer")

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Acked-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>