* libc/argz/argz_add_sep.c (argz_add_sep): Handle empty string

	* libc/argz/argz_add_sep.c (argz_add_sep): Handle empty string
	argument.
	* libc/argz/argz_append.c (argz_append): Handle empty buf argument.
	* libc/argz/argz_create_sep.c (argz_create_sep): Return (NULL, 0)
	on empty input strings.
	* libc/argz/argz_extract.c (argz_extract): Check argz_len before
	looping through argz.
	* libc/argz/argz_stringify.c (argz_stringify): Ditto.