Don't apply product discount if price is already 0

Closes #8725

includes/class-wc-coupon.php

@@ -634,6 +634,7 @@ class WC_Coupon {
 
 		// Handle the limit_usage_to_x_items option
 		if ( $this->is_type( array( 'percent_product', 'fixed_product' ) ) ) {
+			if ( $discounting_amount ) {
 				if ( '' === $this->limit_usage_to_x_items ) {
 					$limit_usage_qty = $cart_item_qty;
 				} else {
@@ -646,6 +647,7 @@ class WC_Coupon {
 					$discount = ( $discount / $cart_item_qty ) * $limit_usage_qty;
 				}
 			}
+		}
 
 		$discount = round( $discount, WC_ROUNDING_PRECISION );