mv643xx_eth: potential null dereference

We assume that "pd" can be null on the previous line, and throughout the
function so we should check it here as well.  This was introduced by
9b2c2ff7 "mv643xx_eth: use sw csum for big packets"

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: David S. Miller <davem@davemloft.net>