p2p/enode: close sources added after FairMix shutdown (#35298)

`FairMix.AddSource` takes ownership of the supplied iterator, but if the
mixer has already been closed it currently returns without closing that
iterator. Close the rejected source in this shutdown path so callers
racing with `Close` do not leak iterator resources.

p2p/enode/iter.go

@@ -356,6 +356,7 @@ func (m *FairMix) AddSource(it Iterator) {
 	defer m.mu.Unlock()
 
 	if m.closed == nil {
+		it.Close()
 		return
 	}
 	m.wg.Add(1)