FilterLayer may produce empty empty blob and lead to crash

[prclibo]

Hi,
When a empty selection mask is ported to FilterLayer, it will produces empty tops. I found this empty tops may cause several issues.

1. As mentioned in skip empty top in backward propagation in filter layer #2569, https://github.com/BVLC/caffe/blob/master/src/caffe/layers/filter_layer.cu#L38 this line will be divided by 0.
2. Many loss layers (e.g. EuclideanLoss) will produces NaN since they divide the summation loss by N, the input blob size.
3. GPU operations (e.g. add_kernel for caffe_gpu_add) will fail because they use CAFFE_GET_BLOCKS(N) and when N == 0 this produces 0 blocks, which makes CUDA unhappy.

So maybe there should be some check added to handle empty blob case generated by FilterLayer

[bhack]

This was a known problem. But maintainers ignored our advice to review and merge #2053 with filterlayer.
/cc @mtamburrano

[bhack]

@BVLC ping