fix a performance bug in drracket's REPL IO.

This one of those classic n^2-loop-that-should-be-linear bug. The fix
speeds up this program (when run in DrRacket) by about 1.75x:

  (for ([x (in-range 10000)])
    (display "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz\n"))

The bug is fairly old too: I introduced this bug on March 6th,
2004. (Thanks, Sam, for making that old history accessible!)