net: sxgbe: fix potential NULL dereference in sxgbe_rx()

In the Linux kernel, the following vulnerability has been resolved: net: sxgbe: fix potential NULL dereference in sxgbe_rx() Currently, when skb is null, the driver prints an error and then dereferences skb on the next line. To fix this, let's add a 'break' after the error message to switch to sxgbe_rx_refill(), which is similar to the approach taken by the other drivers in this particular case, e.g. calxeda with xgmac_rx(). Found during a code review.