fix: more defensive approach to stopping peer in PEX

This helps with some flaky tests, and probably can eliminate some edge cases in real world.

Author: tzdybal

p2p/pex/pex_reactor.go

@@ -268,7 +268,9 @@ func (r *Reactor) Receive(e p2p.Envelope) {
 			r.SendAddrs(e.Src, r.book.GetSelectionWithBias(biasToSelectNewPeers))
 			go func() {
 				// In a go-routine so it doesn't block .Receive.
-				r.Switch.StopPeerGracefully(e.Src, r.String())
+				if r.Switch != nil && r.Switch.IsRunning() {
+					r.Switch.StopPeerGracefully(e.Src, r.String())
+				}
 			}()
 
 		} else {