spidernet: fix racy double-free of skb

It appears that under certain circumstances, a race will result
in a double-free of an skb. This patch null's out the skb pointer
upon the skb free, avoiding the inadvertent deref of bogus data.
The next patch fixes the actual race.

Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Cc: Jens Osterkamp <Jens.Osterkamp@de.ibm.com>
Cc: Kou Ishizaki <kou.ishizaki@toshiba.co.jp>
Signed-off-by: Jeff Garzik <jeff@garzik.org>