A check for vif is made in vnt_interrupt_work.

There is a small chance of leaving interrupt disabled while vif
is NULL and the work hasn't been scheduled.
Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
CC: stable@vger.kernel.org # v4.2+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>