SpatialMaxPooling failure case

Fails for the case:

``` lua
m=nn.SpatialMaxPooling(2,2,2,2):cuda()
input=torch.randn(128,1024,12,12):cuda()
m:forward(input)
```

```
error in SpatialMaxsampling.updateOutput: invalid argument
luajit: ...ch-distro/install/share/lua/5.1/nn/SpatialMaxPooling.lua:18: aborting
```
