gpg: Fix false negatives in Ed25519 signature verification.

* g10/pkglue.c (pk_verify): Fix Ed25519 signatrue values.
* tests/openpgp/verify.scm (msg_ed25519_rshort): New
(msg_ed25519_sshort): New.
("Checking that a valid Ed25519 signature is verified as such"): New.

About one out of 256 signature won't verify due to stripped zero
bytes. See the source comment for details.

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