index-pack.c: correctly initialize appended objects

index-pack.c: correctly initialize appended objects

When index-pack completes a thin pack it appends objects to the pack.
Since the commit 92392b4(index-pack: Honor core.deltaBaseCacheLimit when
resolving deltas) such an object can be pruned in case of memory
pressure, and will be read back again by get_data_from_pack().  For this
to work, the fields in object_entry structure need to be initialized
properly.

Noticed by Pierre Habouzit.

Signed-off-by: Björn Steinbrink <B.Steinbrink@gmx.de>
Acked-by: Nicolas Pitre <nico@cam.org>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>