Start receiver thread in LwIP after initialization

Otherwise the receiver thread might access variables not initialized properly,
and thereby raising pagefaults.

Fixes #802

libports/src/lib/lwip/platform/nic.cc

@@ -263,7 +263,6 @@ extern "C" {
 		/* Setup receiver thread */
 		Nic_receiver_thread *th = new (env()->heap())
 			Nic_receiver_thread(nic, netif);
-		th->start();
 
 		/* Store receiver thread address in user-defined netif struct part */
 		netif->state      = (void*) th;
@@ -285,6 +284,8 @@ extern "C" {
 		for(int i=0; i<6; ++i)
 			netif->hwaddr[i] = _mac.addr[i];
 
+		th->start();
+
 		return ERR_OK;
 	}
 } /* extern "C" */