SELinux: Bug fix in polidydb_destroy

This patch fixes two bugs in policydb_destroy. Two list pointers
(policydb.ocontexts[i] and policydb.genfs) were not being reset to NULL when
the lists they pointed to were being freed. This caused a problem when the
initial policy load failed, as the policydb being destroyed was not a
temporary new policydb that was thrown away, but rather was the global
(active) policydb. Consequently, later functions, particularly
sys_bind->selinux_socket_bind->security_node_sid and
do_rw_proc->selinux_sysctl->selinux_proc_get_sid->security_genfs_sid tried
to dereference memory that had previously been freed.

Signed-off-by: Chad Sellers <csellers@tresys.com>
Signed-off-by: James Morris <jmorris@namei.org>