IB/iser: Handle iser_device allocation error gracefully

"iser_device" allocation failure is "handled" with a BUG_ON() right
before dereferencing the NULL-pointer - fix this!

Signed-off-by: Arne Redlich <arne.redlich@xiranet.com>
Signed-off-by: Erez Zilber <erezz@voltaire.com>