Protect peel_ref fallback case from NULL parse_object result

If the SHA-1 we are requesting the object for does not exist in
the object database we get a NULL back.  Accessing the type from
that is not likely to succeed on any system.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>