iscsi_initiator(4): Fix use-after-free, double-free

ism_stop() already destroys and frees 'sp', including a call to ic_destroy().
Don't dereference 'sp' after ism_stop() and don't invoke ic_destroy() on the
freed memory either.

Reported by: Coverity
CIDs: 1006109, 1304861
Sponsored by: EMC / Isilon Storage Division