Fix gas crash with unreasonably long lines

Fix gas crash with unreasonably long lines

	gas/
	* input-scrub.c (partial_size): Make size_t.
	(buffer_length): Likewise.  Adjust meaning.
	(struct input_save): Adjust partial_size type.
	(input_scrub_reinit): New.
	(input_scrub_push, input_scrub_begin): Use it.
	(input_scrub_next_buffer): Fix buffer extension logic. Only scan
	newly read buffer for newline.