[SCSI] iscsi_tcp: check for valid session before accessing

Check that the session is setup before accessing its
connection. This fixes a oops where userspace tries
to get the ip address before the session is bound to
a host.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>