Your sqlfiddle was confusing. Please don't provide sample data here and then use different sample data in the sqlfiddle. And your desired result here is wrong, since you said in the description
If Previous Row of PO_ID and and Product Name is Same as current Row PO_ID and Product then Quantity of current row should be zero
Anyway, used my own...
select
t.*,
if(@previd = po_id and @prevprod = Product, 0, Quantity) AS new_quantity,
@previd := po_id,
@prevprod := product
from
t
, (select @previd:=null, @prevprod:=null) var_init
order by po_id, product
Note, that the order in the select clause is important, as well as the order by clause.