name-rev: Fix segmentation fault when using --all

name-rev: Fix segmentation fault when using --all

In commit da2478db "describe --always: fall back to showing an
abbreviated object name" we lost the check that skips empty entries in
the object hash table when iterating over it in cmd_name_rev. That may
cause a NULL pointer being handed to show_name(), leading to a
segmentation fault. So add that check back again.

Signed-off-by: Björn Steinbrink <B.Steinbrink@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>