Be careful when dereferencing tags.

Be careful when dereferencing tags.

One caller of deref_tag() was not careful enough to make sure
what deref_tag() returned was not NULL (i.e. we found a tag
object that points at an object we do not have).  Fix it, and
warn about refs that point at such an incomplete tag where
needed.

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