Check that server is nil before calling peers count

On logout happens sometimes that `PeersCount` is called when the server
has been removed.
This commit adds a guard to make sure that the server is not nil when
calling `PeersCount`.

VERSION

@@ -1 +1 @@
-0.62.17
+0.62.18

eth-node/bridge/geth/node.go

@@ -101,5 +101,8 @@ func (w *gethNodeWrapper) RemovePeer(url string) error {
 }
 
 func (w *gethNodeWrapper) PeersCount() int {
+	if w.stack.Server() == nil {
+		return 0
+	}
 	return len(w.stack.Server().Peers())
 }