qla2xxx: Don't crash if we can't find cmd for failed CTIO

In qlt_do_ctio_completion(), there's no point in calling
qlt_term_ctio_exchange() with a NULL cmd -- all that it does is crash
in a NULL pointer dereference, since it does

	qlt_send_term_exchange(vha, cmd, &cmd->atio, 1);

and dereferencing &cmd->atio is a bad idea if cmd itself is NULL.

If we really need to do this, we could take the values from the
failed CTIO we're processing, but it's not clear if it's worth
the replumbing to do that.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>