Fix panic when previewing symlink to ., .., or /
Fix panic when previewing symlink to ., .., or /

The old implementation blindly assumed that a symlink target would have a directory compoment, which the current directory, parent directory, and root directory technically don't have.

Fixes #20.

rhysd/exa,ogham/exa,kbknapp/exa,kbknapp/exa,mneumann/exa,ogham/exa,rhysd/exa,mneumann/exa