buffering of Function accessed with stencil is broken

[FabioLuporini]

The issue is that the buffer halo isn't initialized, so instead of zeros, garbage is read.

    nt = 5
    grid = Grid(shape=(4, 4))

    u = TimeFunction(name='u', grid=grid, space_order=4, save=nt)
    u1 = TimeFunction(name='u', grid=grid, space_order=4, save=nt)

    eqn = Eq(u.forward, u.dx + 1)

    op0 = Operator(eqn, opt='noop')
    op1 = Operator(eqn, opt='buffering')

    op0.apply(time_M=nt-2)
    op1.apply(time_M=nt-2, u=u1)

    assert np.all(u.data == u1.data)