do_for_each_ref: perform the same sanity check for leftovers.

do_for_each_ref: perform the same sanity check for leftovers.

An earlier commit b37a562a added a check to see if the ref
points at a valid object (as a part of 'negative ref' support
which we currently do not use), but did so only while iterating
over both packed and loose refs, and forgot to apply the same
check while iterating over the remaining ones.

We might want to replace the "if null then omit it" check with
"eh --- what business does a 0{40} value have here?" complaint
later since we currently do not use negative refs, but that is
a separate issue.

Signed-off-by: Junio C Hamano <junkio@cox.net>