Allow only preset options for number of posts per page

Dear author,

currently you can set any number of posts per page via a request and/or cookie which can be used to overload big sites with the parameter set to -1. The solution would be to check if $per_page in loop_shop_per_page is in $products_per_page_options array.

Example solution (needs testing & review):

public function loop_shop_per_page( $per_page ) {
	$per_page_default = intval( get_option( 'wppp_default_ppp', '12' ) );
	$products_per_page_options = explode( ' ', apply_filters( 'wppp_products_per_page', get_option( 'wppp_dropdown_options' ) ) );

	if ( isset( $_REQUEST['wppp_ppp'] ) ) :
		$per_page = intval( $_REQUEST['wppp_ppp'] );
	elseif ( isset( $_REQUEST['ppp'] ) ) :
		$per_page = intval( $_REQUEST['ppp'] );
	elseif ( isset( $_COOKIE['woocommerce_products_per_page'] ) ) :
		$per_page = intval( $_COOKIE['woocommerce_products_per_page'] );
	else :
		$per_page = $per_page_default;
	endif;

	if (!in_array($per_page, $products_per_page_options) ) :
		$per_page = $per_page_default;
	endif;

	return $per_page;

}

Please consider implementing this limitation.

Best regards,
Andrej