fix failing case like [[[[]]]]
fix failing case like [[[[]]]]

jnishi/chainer,ktnyt/chainer,chainer/chainer,wkentaro/chainer,wkentaro/chainer,hvy/chainer,pfnet/chainer,cupy/cupy,jnishi/chainer,niboshi/chainer,cupy/cupy,delta2323/chainer,cupy/cupy,hvy/chainer,ysekky/chainer,okuta/chainer,ktnyt/chainer,ronekko/chainer,rezoo/chainer,niboshi/chainer,hvy/chainer,chainer/chainer,ktnyt/chainer,jnishi/chainer,wkentaro/chainer,hvy/chainer,chainer/chainer,cupy/cupy,keisuke-umezawa/chainer,okuta/chainer,tkerola/chainer,niboshi/chainer,niboshi/chainer,wkentaro/chainer,kashif/chainer,jnishi/chainer,chainer/chainer,okuta/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,okuta/chainer,ktnyt/chainer,kiyukuta/chainer,aonotas/chainer,anaruse/chainer