fix #533

fix error in form if pse.quantity = 0 and PSE_CHECK_AVAILABILITY = 0
$pse.quantity.val() is initialized to 0

and fix in category page
if PSE_CHECK_AVAILABILITY = 1 and stock = 0  display 'view product'
button
Merge pull request #641 from zzuutt/fix-'check_available_stock'

fix #533