ssh/agent: validate ed25519 private key length in Add

insertKey and insertCert assume an ed25519.PrivateKey is exactly
64 bytes long: a 32-byte seed followed by a 32-byte public key.
The expression []byte(k)[32:] panics with "slice bounds out of
range" when a shorter slice is passed, which is reachable through
the exported Add API and crashes the caller process.

Reject ed25519 keys whose length is not ed25519.PrivateKeySize in
all four branches (value and pointer variants of insertKey and
insertCert), using the same error wording already used by
parseEd25519Cert in server.go.

This issue was found during a security audit by NCC Group
Cryptography Services, sponsored by Teleport, and was assessed
and is being fixed as a non-security bug.

Change-Id: I4a053835588aad4c3482fe1ca8045542cd960500
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/782423
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>