Fix some sparse warnings

Sparse issues some "Using plain integer as NULL pointer" warnings.
Each warning relates to the use of an '{0}' initialiser expression
in the declaration of an 'struct object_info'. The first field of
this structure has pointer type. Thus, in order to suppress these
warnings, we replace the initialiser expression with '{NULL}'.

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