selinux: fix GPF on invalid policy

levdatum->level can be NULL if we encounter an error while loading
the policy during sens_read prior to initializing it.  Make sure
sens_destroy handles that case correctly.

Reported-by: syzbot+6664500f0f18f07a5c0e@syzkaller.appspotmail.com
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/ss/policydb.c

@@ -732,6 +732,7 @@ static int sens_destroy(void *key, void *datum, void *p)
 	kfree(key);
 	if (datum) {
 		levdatum = datum;
+		if (levdatum->level)
 			ebitmap_destroy(&levdatum->level->cat);
 		kfree(levdatum->level);
 	}