main.c: Forbid sharing non-existing files

So far, slcl would allow to share literally any directory or file, even
if they did not exist, as long as valid credentials were given.

Now, directories cannot be shared, since this is already restricted by
the web interface. This is now considered an invalid request.

On the other hand, attempting to share non-existing files shall now
return a 404 Not Found response to the user.

0 files changed, 0 insertions, 0 deletions