[netlabs #470] io.ops and all io routines in core.ops bomb on NULL strings

[Steve Fink]

For example:

  op puts(in STR) {
    if (($1) && string_length($1)) {
      PIO_write(interpreter, PIO_STDOUT(interpreter), ($1)->bufstart,
                          string_length($1));
    }
    goto NEXT();
  }

Everywhere else the code treats NULL as if it were the empty string.