Fix pointer cast for MemoryPool::Free exposure to Python
Fix pointer cast for MemoryPool::Free exposure to Python

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