Move to the next ASCIIZ string in a list of ASCIIZ strings.

If a segment register is not provided then ES is assumed.

An example of an ASCIIZ list is the DOS environment table. It is usually multiple ASCIIZ strings following one after another. Then eventually the list is terminated by a NULL string.

Example:

    StrNext es:di        ; Update ES:DI to the next ASCIIZ string in the list