Honour the $no_decimals parameter of wpsc_the_product_price()

Added the __return_zero() function to the wpsc_modify_decimals filter
if $no_decimals is true.

Fixes #1432.

Let me know if anyone thinks there will be problems with this approach. The `wpsc_modify_decimals` filter is applied in `wpsc_currency_display()`, which in turn is called by `wpsc_product_variation_price_from()`