ubsan: logical_input_line signed integer overflow

To avoid a completely useless fuzzing ubsan "bug" report, I decided to
make logical_input_line unsigned.

	* input-scrub.c (logical_input_line): Make unsigned.
	(struct input_save): Here too.
	(input_scrub_reinit, input_scrub_close, bump_line_counters),
	(as_where): Adjust to suit.