do_one_ref(): null_sha1 check is not about broken ref

do_one_ref(): null_sha1 check is not about broken ref

f8948e2 (remote prune: warn dangling symrefs, 2009-02-08) introduced a
more dangerous variant of for_each_ref() family that skips the check for
dangling refs, but it also made another unrelated check optional by
mistake.

The check to see if a ref points at 0{40} is not about brokenness, but is
about a possible future plan to represent a deleted ref by writing 40 "0"
in a loose ref when there is a stale version of the same ref already in
.git/packed-refs, so that we can implement deletion of a ref without
having to rewrite the packed refs file excluding the ref being deleted.

This check has to live outside of the conditional.

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