receive-pack: fix "borrowing from alternate object store" implementation

receive-pack: fix "borrowing from alternate object store" implementation

In the alternate_object_database structure, ent->base[] is a buffer the
users can use to form pathnames to loose objects, and ent->name is a
pointer into that buffer (it points at one beyond ".git/objects/").  If
you get a call to add_refs_from_alternate() after somebody used the entry
(has_loose_object() has been called, for example), *ent->name would not be
NUL, and ent->base[] won't be the path to the object store.

This caller is expecting to read the path to the object store in ent->base[];
it needs to NUL terminate the buffer if it wants to.

Signed-off-by: Junio C Hamano <gitster@pobox.com>