Fix file-blob comparison

[alexperovich]

To double check:

• The right tests are in and and the right validation has happened. Guidance: https://github.com/dotnet/core-eng/tree/master/Documentation/Validation

[epananth]

Can we do a test against any of the already existing builds?

[MattGal]

seems like it fixes some bug that repros when the exact file size is a multiple of 1 MB, but where is the bug this is fixing?

[epananth]

While we assigned the existing build to different channel we saw some error -> https://dev.azure.com/dnceng/internal/_build/results?buildId=1404086&view=logs&j=ba23343f-f710-5af9-782d-5bd26b102304&t=6e277ba4-1c1e-552d-b96f-db0aeb4be20a

[MattGal]

Now that I've seen the failure being fixed I'm fairly convinced it'll fix it; please consider actually filing that bug for posterity's sake.

[epananth]

Created the issue -> #8004

[alexperovich]

Can we do a test against any of the already existing builds?

I tested this against the failing build I had.

[alexperovich]

seems like it fixes some bug that repros when the exact file size is a multiple of 1 MB, but where is the bug this is fixing?

This is a bug that always happens when this code path happens. The loop continues until the Read returns 0 indicating EOF, but the call to read 0 from the blob still happens and fails.