Do not unnecessarily refresh in config menu

We don't need to refresh windows every time an unassigned key is
pressed. Add a condition to skip the refresh part if that is the case.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.c

@@ -283,6 +283,8 @@ main (int argc, char **argv)
                 case 'S':
                   custom_sidebar_config ();
                   break;
+                default:
+                  continue;
                 }
               wins_reset ();
               wins_update ();