Make brew doctor (and prune) robust in the face of really bad symlinks.

`ArgumentError` is thrown when Ruby's pathname lib hits a symlink with
binary zeros in it ('\0').  These links have shown up in
/usr/local/bin for a couple users (#18257,
https://gist.github.com/barttenbrinke/2948629 and others).  As a
result, caused `brew doctor`/`brew prune` to fail and exit mid-run.

`brew doctor` will now report those links as bad, and `brew prune`
will remove them.