pack-objects: don't check size when the object is bad

pack-objects: don't check size when the object is bad

sha1_object_info() in check_objects() may fail to locate an object in
the pack and return type OBJ_BAD. In that case, it will likely leave
the "size" field untouched. We delay error handling until later in
prepare_pack() though. Until then, do not touch "size" field.

This field should contain the default value zero, but we can't say
sha1_object_info() cannot damage it. This becomes more important later
when the object size may have to be retrieved back from the
(non-existing) pack.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>