UnicastZenPing's PingingRound should prevent opening connections afte…

…r being closed

This may cause them to leak. Provisioning for it was made in #22277 but sadly a crucial ensureOpen call was forgotten

core/src/main/java/org/elasticsearch/discovery/zen/UnicastZenPing.java

@@ -385,12 +385,14 @@ public Connection getOrConnect(DiscoveryNode node) throws IOException {
             try (Releasable ignore = connectionLock.acquire(node.getAddress())) {
                 result = tempConnections.get(node.getAddress());
                 if (result == null) {
+                    ensureOpen();
                     boolean success = false;
                     result = transportService.openConnection(node, connectionProfile);
                     try {
                         transportService.handshake(result, connectionProfile.getHandshakeTimeout().millis());
                         synchronized (this) {
-                            // acquire lock to prevent concurrent closing
+                            // acquire lock and check if closed, to prevent leaving an open connection after closing
+                            ensureOpen();
                             Connection existing = tempConnections.put(node.getAddress(), result);
                             assert existing == null;
                             success = true;