Check for existance of the directory in wip.

If a buffer edit a file in an non existing directory
magit-inside-worktree-p fill fail with an error. As
turn-on-magit-wip-save is meant to be in post-command-hook, it must not
fail with an error, so we check if the directory exists.
Merge pull request #1564 from vanicat/t/turn-on-should-no-fail

Check for existance of the directory in wip.