When index-pack completes a thin pack it appends objects to the pack.
Since the commit 92392b4a(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: NBjörn Steinbrink <B.Steinbrink@gmx.de>
Acked-by: NNicolas Pitre <nico@cam.org>
Acked-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>