gas: input_scrub buffers

This tidies freeing of input_scrub buffers on failure paths, making
input_scrub_end iterate over any input_scrub_push'd files or string
buffers to clean up memory.

	* input-scrub.c (input_scrub_free): New function.
	(input_scrub_pop): Call it rather than input_scrub_end.
	(input_scrub_end): Iterate over next_saved_file freeing
	buffers.
	(input_scrub_next_buffer): Move sb_kill to input_scrub_free.