net: Reset pbuf length when re-queueing on error.

I recently pulled a NXP crash fix for their ethernet driver which will
requeue a pbuf to the ethernet driver rather than sending it to the
lwip stack if it can't allocate a new pbuf to keep the ethernet
hardware primed with available packet buffers.  While recently
reviewing this code I noticed that the full size of the pbuf wasn't
used on this re-queueing operation but the size of the last received
packet.  I now reset the pbuf size back to its originally allocated
size before doing this requeue operation.

Author: adamgreen

libraries/net/eth/lwip-eth/arch/lpc17_emac.c

@@ -346,6 +346,7 @@ static struct pbuf *lpc_low_level_input(struct netif *netif)
 	struct lpc_enetdata *lpc_enetif = netif->state;
 	struct pbuf *p = NULL;
 	u32_t idx, length;
+	u16_t origLength;
 
 #ifdef LOCK_RX_THREAD
 #if NO_SYS == 0
@@ -428,6 +429,7 @@ static struct pbuf *lpc_low_level_input(struct netif *netif)
 
 			/* Zero-copy */
 			p = lpc_enetif->rxb[idx];
+			origLength = p->len;
 			p->len = (u16_t) length;
 
 			/* Free pbuf from descriptor */
@@ -440,6 +442,7 @@ static struct pbuf *lpc_low_level_input(struct netif *netif)
     			LINK_STATS_INC(link.drop);
 
     			/* Re-queue the pbuf for receive */
+    			p->len = origLength;
     			lpc_rxqueue_pbuf(lpc_enetif, p);
 
     			LWIP_DEBUGF(UDP_LPC_EMAC | LWIP_DBG_TRACE,