Fix overflow in Col2Im
Fix overflow in Col2Im

pfnet/chainer,niboshi/chainer,chainer/chainer,ktnyt/chainer,jnishi/chainer,ktnyt/chainer,chainer/chainer,tkerola/chainer,hvy/chainer,okuta/chainer,wkentaro/chainer,wkentaro/chainer,keisuke-umezawa/chainer,ktnyt/chainer,hvy/chainer,keisuke-umezawa/chainer,chainer/chainer,chainer/chainer,niboshi/chainer,keisuke-umezawa/chainer,wkentaro/chainer,hvy/chainer,niboshi/chainer,jnishi/chainer,okuta/chainer,keisuke-umezawa/chainer,okuta/chainer,jnishi/chainer,niboshi/chainer,okuta/chainer,hvy/chainer,ktnyt/chainer,jnishi/chainer,wkentaro/chainer