Fix handle cleanup in CommandHandler. (#34262)

When a CommandHandler went away, it invalidated its handles, but did not remove
them from the list.  This would cause it to fail a fatal assert when the
non-empty list was destroyed.

The fix is to clear the list once we have invalidated all the handles, since we
no longer need to track them once they have been invalidated.

Author: bzbarsky-apple

src/app/CommandHandlerImpl.cpp

@@ -323,6 +323,7 @@ void CommandHandlerImpl::InvalidateHandles()
     {
         handle->Invalidate();
     }
+    mpHandleList.Clear();
 }
 
 void CommandHandlerImpl::IncrementHoldOff(Handle * apHandle)