infinite loop in repl after lexical error

[will]

This bug was reported by Jed Davis.

If a control character (other than the usual CR/LF/HT/&c.) is entered into the REPL, for example because I accidentally hit an arrow key or control-P or some such, the result is an infinite loop of this:

  Error: get-datum: Lexical Error: Incomplete or illegal token: INPUT PORT *console-input*>
  Computation is not continuable.
  debug> 
  Error: get-datum: Lexical Error: Incomplete or illegal token: INPUT PORT *console-input*>
  Computation is not continuable.
  debug> 
  Error: get-datum: Lexical Error: Incomplete or illegal token: INPUT PORT *console-input*>
  Computation is not continuable.
  debug> 

which needs a SIGQUIT to terminate (i.e., SIGINT does nothing).

It would be more helpful to consume the offending character before complaining of it, to ensure a finite number of errors on finite input.

[will]

Fixed by changeset:5112.