Clustering sometimes gets NaN

Originally reported by Hiroshi Kuwajima's [post](https://groups.google.com/d/msg/jubatus/J8dDkXKj7JE/S8hE2HbM0v0J) 

There are devide-by-zero and log(0) calculation exists.
It appears values with limited distribution(like integer).
For first aid, it should use very small value instead of zero.

Some radical(or mathematical) solution will be needed further.