tdf#125030: Prevent double-clearing the guard

... discovered after after commit d38f9934.

Change-Id: I09bcee0053d1a5132483c933a903f006a236a679
Reviewed-on: https://gerrit.libreoffice.org/71550
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>

svl/source/undo/undo.cxx

@@ -871,6 +871,7 @@ bool SfxUndoManager::Repeat( SfxRepeatTarget &rTarget )
         aGuard.clear();
         if ( pAction->CanRepeat( rTarget ) )
             pAction->Repeat( rTarget );
+        aGuard.reset(); // allow clearing in guard dtor
         return true;
     }