receive-pack: do not expect object 0{40} to exist

receive-pack: do not expect object 0{40} to exist

When pushing to delete a ref, it uses 0{40} as an object name to signal
that the request is a deletion. We shouldn't trigger "deletion of a
corrupt ref" warning in such a case, which was designed to notice that a
ref points at an object that is truly missing from the repository.

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