Update ethernetif.c

if device->eth_rx is NULL ,rx thread will hard fault...

components/net/lwip-1.4.1/src/netif/ethernetif.c

@@ -367,6 +367,8 @@ static void eth_rx_thread_entry(void* parameter)
             /* receive all of buffer */
             while (1)
             {
+            	if(device->eth_rx == RT_NULL) break;
+            	
                 p = device->eth_rx(&(device->parent));
                 if (p != RT_NULL)
                 {