Return err in copy() fstat cb, because stat could be undefined or null

[ulikoehler]

I had a problem with FlyLaTeX, where some weird arguments (notexistent files etc) were passed to copy() --> err is ENOENT, but the copy() code failed to properly terminate the lstat callback when err is not false-ish, resulting in the whole application terminating with this stacktrace:

/var/lib/nodejs/node-apps/flylatex/node_modules/fs-extra/lib/copy.js:41
    if (stats.isDirectory()) {

This PR fixes this by not only calling callback(err) but instead returning it. I didn't check other functions or files for similar problems. Please feel free to comment if you think this should be fixed otherwise or see possible side-effects with this fix.

[jprichardson]

Would it be possible for you to add a regression test? i.e. a test demonstrating a fail condition before the patch, and then when your patch is applied, the test passes?

[ulikoehler]

Sure! I'll submit another pull request to be able to see the difference easily.

[jprichardson]

Excellent! Thanks. I should be able to review it this evening or by tomorrow morning at the latest.

[ulikoehler]

Sure, take your time. I suppose flylatex won't upgrade to the newest fs-extra version anyway, so there's no need to hurry!

When I have some time, I'll check if there could be a similar error in functions other than copy().

[ulikoehler]

It seems like this was the only place where the code didn't return when err is not false-ish.

[jprichardson]

Cool, good to know :) Thanks again.

[ulikoehler]

Thanks for merging ;-) Sorry if the report was a bit confusing, I was really tired at that point, after debugging FlyLaTeX for hours :)