Minor fix of sparse_matmul
Minor fix of sparse_matmul

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