kbx: Minor cleanup for the previous fix.

* kbx/keybox-search.c (blob_get_keyid): Rename to
blob_get_first_keyid. Check number of keys and remove blob type check.

There is no need to check the blob type. We already know that it is a
key blob type and keyids are used for X.509 and OpenPGP. Also added
check for number of keys because the other parser functions do it as
well.

• Signed-off-by: Werner Koch <wk@gnupg.org>