t_list.c/listTypeDelete: mind for the edge case of ziplist

When deleting the last one of a list of ziplist type.
If the direction is REDIS_TAIL the next, li->zi is set to ZIP_END, but logically it should be NULL.

This cause the li run another more time for ZIP_END. 
In t_list.c the problem doesn't appear, because listTypeDelete deal with the ZIP_END condition. :-)