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

sys/dev/iscsi_initiator/iscsi.c

@@ -804,8 +804,6 @@ iscsi_stop(void)
      TAILQ_FOREACH_SAFE(sp, &isc->isc_sess, sp_link, sp_tmp) {
 	  //XXX: check for activity ...
 	  ism_stop(sp);
-	  if(sp->cam_sim != NULL)
-	       ic_destroy(sp);
      }
      mtx_destroy(&isc->isc_mtx);
      sx_destroy(&isc->unit_sx);