Samples: Bluetooth: Checking return value of functions

Without this, an assert in gatt.c:5350 could happen.
It happened when the link got disconnected during
service discovery procedure.

samples/bluetooth/central_uart/src/main.c

@@ -289,14 +289,29 @@ static int uart_init(void)
 static void discovery_complete(struct bt_gatt_dm *dm,
 			       void *context)
 {
+	int err;
 	struct bt_nus_client *nus = context;
 	LOG_INF("Service discovery completed");
 
 	bt_gatt_dm_data_print(dm);
 
-	bt_nus_handles_assign(dm, nus);
-	bt_nus_subscribe_receive(nus);
+	err = bt_nus_handles_assign(dm, nus);
+	if (err)
+	{
+		bt_gatt_dm_data_release(dm);
+		LOG_INF("bt_nus_handles_assign failed with errorcode %i", err);
+		goto cleanup;
+	}
+
+	err = bt_nus_subscribe_receive(nus);
+	if (err)
+	{
+		bt_gatt_dm_data_release(dm);
+		LOG_INF("bt_nus_subscribe_receive failed with errorcode %i", err);
+		goto cleanup;
+	}
 
+	cleanup:
 	bt_gatt_dm_data_release(dm);
 }