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.
Cherry-picked from master branch.
Signed-off-by: Thomas Hochstein <thh@inter.net>
307f1c3dad