Fix another bug in the Dragon4 code (the bug exists in the original

Fix another bug in the Dragon4 code (the bug exists in the original
paper).  The bug caused the following behavior:

    (fluid-let ((flonum-unparser-cutoff '(absolute 2 normal)))
      (number->string 0.005))
    ;Value 3: ".01"

    (fluid-let ((flonum-unparser-cutoff '(absolute 2 normal)))
      (number->string 0.00499))
    ;Value 4: ".005"

The problem is that in the second case the trailing digit "5" should
not be generated.  The fix works by preventing any digits being output
by the digit-generation loop when the first digit to be output would
be to the right of the cutoff point.