Fix bug with od/hd/hexdump. "*" lines are supposed to indicate one or
duplicates of the previous line, but a small file with less then 16
characters of zeros in it will be falsy identified as a repeat of
the (non-existant) previous line.  i.e. the first line of output winds
up being a "*".  Added a bit of code to handle the degenerate 'there is
no previous line' case for the first line.