cat-file --batch / --batch-check: do not exit if hashes are missing

cat-file --batch / --batch-check: do not exit if hashes are missing

Previously, cat-file --batch / --batch-check would silently exit if it
was passed a non-existent SHA1 on stdin.  Now it prints "<SHA1>
missing" as in all other cases (and as advertised in the
documentation).

Note that cat-file --batch-check (but not --batch) will still output
"error: unable to find <SHA1>" on stderr if a non-existent SHA1 is
passed, but this does not affect parsing its stdout.

Also, type <= 0 was previously using the potentially uninitialized
type variable (relying on it being 0); it is now being initialized.

Signed-off-by: Lea Wiemann <LeWiemann@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>