crypto/x509: don't fail to parse addition elements in GeneralSubtree.

crypto/x509: don't fail to parse addition elements in GeneralSubtree.

The GeneralSubtree structure can have additional elements after the name
(minimum and maximum, which are unused). Previously these fields, if
present, would cause a parse error. This change allows trailing data in
the GeneralSubtrees structure.

Change-Id: I6bfb11ec355fa6812810a090c092a5ee0fdeddc3
Reviewed-on: https://go-review.googlesource.com/77333
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>