Fix segmentation fault on EAP-TTLS phase 2 EAP method init failure

This is based on a patch and report by Masashi Honma
<honma@ictec.co.jp>. The issue is more generic than just TNC, though,
since failure to initialize any phase 2 EAP method can result in NULL
dereference.

src/eap_server/eap_ttls.c

@@ -954,7 +954,7 @@ static int eap_ttls_phase2_eap_init(struct eap_sm *sm,
 	sm->init_phase2 = 1;
 	data->phase2_priv = data->phase2_method->init(sm);
 	sm->init_phase2 = 0;
-	return 0;
+	return data->phase2_priv == NULL ? -1 : 0;
 }