Fix div-by-zero errors on files which don't have any changed content

[Obi-Dann]

Copy of Squirrel#1361

There are cases where the length in write operations becomes zero, at which point this code will throw an exception and fail to create bsdiff deltas.

As this is critical to our production workflow, it has been temporarily published as ppy.Squirrel.windows (along with several other PRs that are not merged into official releases yet).

Test files which cause the issue to reproduce (large): https://www.dropbox.com/sh/p13oiqir02xzny1/AABUHNZeP2u2T-AjViFYCG6na?dl=0