Also detect single dashes as error

This also recognizes single dashes as invalid input:

    rsync_tmbackup.sh - 1 2 3

Expected output: an error because it's unknown what the single `-` could mean

Actual output:

    rsync_tmbackup: Safety check failed - the destination does not appear to be a backup folder or drive (marker file not found).
    rsync_tmbackup: If it is indeed a backup folder, you may add the marker file by running the following command:
    rsync_tmbackup: 
    rsync_tmbackup: mkdir -p -- "1" ; touch "1/backup.marker"
    rsync_tmbackup:

0 files changed, 0 insertions, 0 deletions