Bail out if output path is the root folder

[catull]

ng serve --output-path=. would wipe out the entire root folder otherwise.
This PR prevent issue #4481 from occurring, this closes #4481.

[hansl]

Seems sensible. Thanks :D

[googlebot]

So there's good news and bad news.

👍 The good news is that everyone that needs to sign a CLA (the pull request submitter and all commit authors) have done so. Everything is all good there.

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[hansl]

Oh CLAbot, you're so silly, I just merge master in this PR before merging it. All good.

[filipesilva]

This should also go in ng build. I'll add a PR for it.

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