Fix free checkout fatal error

* BUG FIX: Fixes an issue for free checkouts, where fatal errors would occur with Paystack as the active gateway.

class.pmprogateway_paystack.php

@@ -493,6 +493,10 @@ static function pmpro_checkout_before_change_membership_level( $user_id, $morder
                         return;
                     }
 
+                    if ( $morder->gateway != "paystack" ) {
+                        return;
+                    }
+
                     if ( empty( $morder->code ) ) {
                         $morder->code = $morder->getRandomCode();
                     }