(captive portal) don't try to generate invalid vouchers, closes...

(captive portal) don't try to generate invalid vouchers, closes https://github.com/opnsense/core/issues/1253

Let's keep this simple for now, just don't leave the dialog as long as the input isn't valid yet....

(cherry picked from commit fc39bb33)

src/opnsense/mvc/app/views/OPNsense/CaptivePortal/vouchers.volt

@@ -178,7 +178,10 @@ POSSIBILITY OF SUCH DAMAGE.
             var voucher_validity = $("#voucher-validity").val();
             var voucher_quantity = $("#voucher-quantity").val();
             var voucher_groupname = $("#voucher-groupname").val();
-
+            if (!$.isNumeric(voucher_validity) || !$.isNumeric(voucher_quantity)) {
+                // don't try to generate vouchers then validity or quantity are invalid
+                return;
+            }
             ajaxCall(url="/api/captiveportal/voucher/generateVouchers/" + voucher_provider + "/",
                     sendData={
                         'count':voucher_quantity,