[SELINUX]: Fix bug in security_sid_mls_copy

The following fixes a bug where random mem is being tampered with in the
non-mls case; encountered by Jashua Brindle on a gentoo box.

Signed-off-by: Venkat Yekkirala <vyekkirala@TrustedCS.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>