Fix issue with numeric overflow.

src/json.json_lexer.m:
	Only return number tokens for finite numbers; return
	an error otherwise.

src/json.writer.m:
	Avoid problems with floor_to_int when its argument will
	not fit into a Mercury int.

	Throw an exception if we try to write a non-finite number.

src/json.m:
	Fix a typo.

	Document that writers throw an exception for non-finite
	numbers.