Prevent renaming ML object into existing file.

Created by: th-h

When renaming objects in the Media Library, s9y didn't check if a file with the same name already exists, resulting in a file name collision deleting both files from the database and from disk.

Add a check to avoid that.

An error message would be nice, too, but that may be added later on.

Tested on s9y-stable test instance.

Fixes #666 (closed). (The number of the beast.)

Signed-off-by: Thomas Hochstein thh@inter.net