copySync, moveSync: remove extra step in reading loop

[kdziamura]

By some reasons on our environment fs.readSync from node 8.x throws error when try to read data from position which more than size of file. Here is the test script with logs in comments:
https://gist.github.com/wontem/da867f66324e1cbe712c717b5fefb944

Since we start using node 8.x and fs-extra 5.x - the issue is disappeared (fs-extra uses copyFileSync from node) but with node 8.x and fs-extra 3.0.1 the issue is reproducible.
While investigating the problem I've noticed that the method copySync from fs-extra 3.0.1 uses extra step in its read loop. To be more safe I want to suggest remove this step from fallback logic for copySync and for moveSync in fs-extra 5.x

[coveralls]

Coverage increased (+0.07%) to 85.565% when pulling 7e2aba0 on wontem:fs-readSync-throws-error into a0d44c1 on jprichardson:master.

[RyanZim]

I'm not understanding exactly why these changes are needed and what this fixes?

[kdziamura]

@RyanZim bytesRead will be greater than zero right after the file is copied.
At this moment pos === srcStat.size and the next line will call fs.readSync with this pos as argument for position.
On my environment node throws error "ENOENT" if try to read file from position which greater than or equal to its size.

[JPeer264]

LGTM

[jprichardson]

Nice work. Is it possible to get a test in place for at least one of these cases? Thanks.

[kdziamura]

@jprichardson Cases I've mentioned are just for environment on our project and they already covered with existed tests. This MR is just for removing extra step in loop.