ibmvnic: Check for NULL skb's in NAPI poll routine

After introduction of commit d0869c00, there were some instances of
RX queue entries from a previous session (before the device was closed
and reopened) returned to the NAPI polling routine. Since the corresponding
socket buffers were freed, this resulted in a panic on reopen. Include
a check for a NULL skb here to avoid this.

Fixes: d0869c00 ("ibmvnic: Clean RX pool buffers during device close")
Signed-off-by: NThomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/ibm/ibmvnic.c

@@ -1901,6 +1901,11 @@ static int ibmvnic_poll(struct napi_struct *napi, int budget)
 			dev_kfree_skb_any(rx_buff->skb);
 			remove_buff_from_pool(adapter, rx_buff);
 			continue;
+		} else if (!rx_buff->skb) {
+			/* free the entry */
+			next->rx_comp.first = 0;
+			remove_buff_from_pool(adapter, rx_buff);
+			continue;
 		}
 
 		length = be32_to_cpu(next->rx_comp.len);