Use key=NULL when clearing PTK with set_key

The key clearing operations are using NULL everywhere else, so make
this consistent with other callers.

src/ap/wpa_auth.c

@@ -1314,8 +1314,7 @@ void wpa_remove_ptk(struct wpa_state_machine *sm)
 {
 	sm->PTK_valid = FALSE;
 	os_memset(&sm->PTK, 0, sizeof(sm->PTK));
-	wpa_auth_set_key(sm->wpa_auth, 0, WPA_ALG_NONE, sm->addr, 0, (u8 *) "",
-			 0);
+	wpa_auth_set_key(sm->wpa_auth, 0, WPA_ALG_NONE, sm->addr, 0, NULL, 0);
 	sm->pairwise_set = FALSE;
 	eloop_cancel_timeout(wpa_rekey_ptk, sm->wpa_auth, sm);
 }