tka: guard against key-length panics when verifying signatures

In late 2022 a subtle but crucial part of documentation was added to ed25519.Verify: It
will panic if len(publicKey) is not [PublicKeySize].

https://cs.opensource.google/go/go/+/02ed0e5e67530e6b041989d55048ce373dc60327

This change catches that error so it won't lead to a panic.

Signed-off-by: Tom DNetto <tom@tailscale.com>
Updates https://github.com/tailscale/corp/issues/8568

tka/key.go

@@ -145,6 +145,9 @@ func signatureVerify(s *tkatype.Signature, aumDigest tkatype.AUMSigHash, key Key
 	//            so we should use the public contained in the state machine.
 	switch key.Kind {
 	case Key25519:
+		if len(key.Public) != ed25519.PublicKeySize {
+			return fmt.Errorf("ed25519 key has wrong length: %d", len(key.Public))
+		}
 		if ed25519consensus.Verify(ed25519.PublicKey(key.Public), aumDigest[:], s.Signature) {
 			return nil
 		}