crypto: fix error check in toECDSA (#15632)

With this change,

    key, err := crypto.HexToECDSA("000000...")
    
returns nil key and an error instead of a non-nil key with nil X
and Y inside. Issue found by @guidovranken.

crypto/crypto.go

@@ -98,6 +98,9 @@ func toECDSA(d []byte, strict bool) (*ecdsa.PrivateKey, error) {
 	}
 	priv.D = new(big.Int).SetBytes(d)
 	priv.PublicKey.X, priv.PublicKey.Y = priv.PublicKey.Curve.ScalarBaseMult(d)
+	if priv.PublicKey.X == nil {
+		return nil, errors.New("invalid private key")
+	}
 	return priv, nil
 }