slocate leaks filenames of protected directories

[steven () masterwebnet com]

* Version tested: 3.1

* Problem description: slocate doesn't check readability bit of containing directory. It can divulge the existence of 
files in a directory that is unreadable (e.g. by the 'ls' command) by a user.

* Demonstration:

As user1:

$ cd /tmp
$ mkdir dir
$ chmod 711 dir
$ cd dir
$ touch "a-secret-file"
$ cd ..

$ updatedb -o db -U dir

As user2:

$ cd /tmp
$ ls dir
ls: .: Permission denied

But:

$ slocate -d db file
dir/a-secret-file