Try to replace:
Code:
<input type="button" value="{$lng.lbl_add_to_cart}" onclick="document.orderform_{$products[product].productid}_{$products[product].add_date}.submit();">
with(not tested):
Code:
{if $product.avail gt 0}
<input type="button" value="{$lng.lbl_add_to_cart}" onclick="document.orderform_{$products[product].productid}_{$products[product].add_date}.submit();">
{else}
{$lng.txt_out_of_stock}
{/if}