selinux: ensure the policy has been loaded before reading the sidtab stats

Check to make sure we have loaded a policy before we query the
sidtab's hash stats.  Failure to do so could result in a kernel
panic/oops due to a dereferenced NULL pointer.

Fixes: 66f8e2f03c ("selinux: sidtab reverse lookup hash table")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/ss/services.c

@@ -1286,6 +1286,12 @@ int security_sidtab_hash_stats(struct selinux_state *state, char *page)
 {
 	int rc;
 
+	if (!state->initialized) {
+		pr_err("SELinux: %s:  called before initial load_policy\n",
+		       __func__);
+		return -EINVAL;
+	}
+
 	read_lock(&state->ss->policy_rwlock);
 	rc = sidtab_hash_stats(state->ss->sidtab, page);
 	read_unlock(&state->ss->policy_rwlock);