crypto/x509: don't ignore asn1.Marshal error

I don't see how the call could fail, so, no test. Just a code cleanup in
case it can fail in the future.

Fixes #15987.

Change-Id: If4af0d5e7d19cc8b13fb6a4f7661c37fb0015e83
Reviewed-on: https://go-review.googlesource.com/23860
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>

src/crypto/x509/x509.go

@@ -1796,6 +1796,9 @@ var oidExtensionRequest = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 9, 14}
 func newRawAttributes(attributes []pkix.AttributeTypeAndValueSET) ([]asn1.RawValue, error) {
 	var rawAttributes []asn1.RawValue
 	b, err := asn1.Marshal(attributes)
+	if err != nil {
+		return nil, err
+	}
 	rest, err := asn1.Unmarshal(b, &rawAttributes)
 	if err != nil {
 		return nil, err