Fix large file overflow.

[IvanVolosyuk]

I've got 22.68 on 300G filesystem image, when maximum entropy is supposed to be 8 :) It looks like it is caused by the counter overflow, probably for number of zeros. This change fixes that, now I'm getting reasonable 3.06 entropy. Also, increase buffer size to speed up large stream processing. 1M should be reasonable size for 2025.