make check_object() resilient to pack corruptions

make check_object() resilient to pack corruptions

The check_object() function tries to get away with the least amount of
pack access possible when it already has partial information on given
object rather than calling the more costly packed_object_info().

When things don't look right, it should just give up and fall back to
packed_object_info() directly instead of die()'ing.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>