selinux: reject an unclaimed class value in security_get_classes()

In the Linux kernel, the following vulnerability has been resolved: selinux: reject an unclaimed class value in security_get_classes() security_get_classes() sizes an array by p_classes.nprim and fills it at value - 1, so a class value the policy never defines leaves a NULL. sel_make_classes() passes every entry to sel_make_dir(), reaching the same d_alloc_name() dereference as the permission array. The class symbol table is allowed to be sparse (policydb_class_isvalid() exists to absorb that), but this getter builds its own array straight from the hash table and has no such predicate. Fail the lookup when a value went unclaimed instead of handing out the NULL. Conforming policies define every class they declare and are unaffected.