Fix #1768: Handle segfault when current directory was unlinked

[WojciechMazur]

Resolves #1768
I've imported fix of avdv@939a129 mentioned in issue.
I have no idea how to provide valid tests for this case. I don't think i can be done using scripted-tests either.

[sjrd]

I have no idea how to provide valid tests for this case. I don't think i can be done using scripted-tests either.

Did you test it manually using the reproducing steps in #1768, at least?

[WojciechMazur]

I have no idea how to provide valid tests for this case. I don't think i can be done using scripted-tests either.

Did you test it manually using the reproducing steps in #1768, at least?

Yes, I've check it out.
Here's stack trace that would be produced

java.nio.file.NoSuchFileException: Could not determine current working directory
        at java.lang.Throwable.fillInStackTrace(Unknown Source)
        at scala.scalanative.nio.fs.UnixException$.apply(Unknown Source)
        at scala.scalanative.nio.fs.UnixFileSystemProvider.getUserDir(Unknown Source)
        at scala.scalanative.nio.fs.UnixFileSystemProvider.fs$lzycompute(Unknown Source)
        at scala.scalanative.nio.fs.UnixFileSystemProvider.getFileSystem(Unknown Source)
        at java.nio.file.FileSystems$.fs$lzycompute(Unknown Source)
        at java.nio.file.FileSystems$.getDefault(Unknown Source)
        at java.nio.file.Paths$.fs$lzycompute(Unknown Source)
        at java.nio.file.Paths$.get(Unknown Source)
        at Test$.main(Unknown Source)
        at <none>.main(Unknown Source)
        at <none>.__libc_start_main(Unknown Source)
        at <none>._start(Unknown Source)

instead of previous

zsh: segmentation fault (core dumped)  ~/projects/scala-native/sandbox/target/scala-2.11/sandbox-out