Avoid computing with uninitialized array
Avoid computing with uninitialized array

`np.empty(...) * 2` may cause
`RuntimeWarning: overflow encountered in multiply`

chainer/chainer,pfnet/chainer,wkentaro/chainer,niboshi/chainer,hvy/chainer,niboshi/chainer,wkentaro/chainer,chainer/chainer,chainer/chainer,hvy/chainer,hvy/chainer,wkentaro/chainer,wkentaro/chainer,chainer/chainer,hvy/chainer,niboshi/chainer,niboshi/chainer