SELinux: fix error code in policydb_init()

If hashtab_create() returns a NULL pointer then we should return -ENOMEM
but instead the current code returns success.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <pmoore@redhat.com>