array size plus one.
	(Stringpool_template::set_string_offsets): Subtract one from key
	before using it as an array index.
	(Stringpool_template::get_offset_with_length): Likewise.
	(Stringpool_template::write_to_buffer): Likewise.
	* stringpool.h (Stringpool_template::get_offset_from_key):
	Likewise.