src/rrd_list.c: fix local directory listing when connection to rrdcached fails

src/rrd_list.c: fix local directory listing when connection to rrdcached fails

        Without this fix, when the remote rrdcached cannot be reached
        and a path that exists on the local machine (such as '/') is
        asked for,  'list' returns 'ls' of the local path.

        Example of erroneous output:

	$ rrdtool list -d rrdcached-host:42222 /
        Error connecting to rrdcached: Unable to connect to rrdcached: Connection refused
        var
        storage
        lib
        sysroot
        rules.d
        lib64
        backup
        opt
        bin
        sbin
        boot
        proc
        usr
        dev
        sys
        srv
        mnt
        run
        etc
        media
        tmp
        home
        root
        ERROR: Unable to connect to rrdcached: Connection refused