crypto: return an error if a hash function is not available

Calling New otherwise panics, which is unnecessary, especially in
crypto.SignMessage and especially if we add a new not-implemented hash
value for ML-DSA external Mu.

Change-Id: I9f8d29d01e126838d7d2585133e562536a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/745660
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>

auth.go

@@ -21,6 +21,9 @@ import (
 // verifyHandshakeSignature verifies a signature against unhashed handshake contents.
 func verifyHandshakeSignature(sigType uint8, pubkey crypto.PublicKey, hashFunc crypto.Hash, signed, sig []byte) error {
 	if hashFunc != directSigning {
+		if !hashFunc.Available() {
+			return fmt.Errorf("hash function unavailable: %v", hashFunc)
+		}
 		h := hashFunc.New()
 		h.Write(signed)
 		signed = h.Sum(nil)

cipher_suites.go

@@ -13,6 +13,7 @@ import (
 	"crypto/rc4"
 	"crypto/sha1"
 	"crypto/sha256"
+	_ "crypto/sha512" // for crypto.SHA384
 	"fmt"
 	"hash"
 	"runtime"