NetLabel: only deref the CIPSOv4 standard map fields when using standard mapping

Fix several places in the CIPSO code where it was dereferencing fields which
did not have valid pointers by moving those pointer dereferences into code
blocks where the pointers are valid.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: James Morris <jmorris@namei.org>