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_test.go

@@ -40,6 +40,15 @@ func TestKeccak256Hash(t *testing.T) {
 	checkhash(t, "Sha3-256-array", func(in []byte) []byte { h := Keccak256Hash(in); return h[:] }, msg, exp)
 }
 
+func TestToECDSAErrors(t *testing.T) {
+	if _, err := HexToECDSA("0000000000000000000000000000000000000000000000000000000000000000"); err == nil {
+		t.Fatal("HexToECDSA should've returned error")
+	}
+	if _, err := HexToECDSA("ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff"); err == nil {
+		t.Fatal("HexToECDSA should've returned error")
+	}
+}
+
 func BenchmarkSha3(b *testing.B) {
 	a := []byte("hello world")
 	for i := 0; i < b.N; i++ {