x509: Fixed ASN.1 encoding in CRL Distribution Points extension

The ASN.1 encoding of the CRL Distribution Points extension showed an invalid false 'IsCompound' which caused a display problem in the Windows certificate viewer.

LGTM=agl
R=agl
CC=golang-codereviews
https://codereview.appspot.com/143320043

Committer: Adam Langley <agl@golang.org>