Fix NPE bug when creating connection group in ConnectionManager

- The NPE will occur when multiple threads are trying to create the connection group for the same namespace

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

sentinel-cluster/sentinel-cluster-server-default/src/main/java/com/alibaba/csp/sentinel/cluster/server/connection/ConnectionManager.java

@@ -55,7 +55,7 @@ public final class ConnectionManager {
         ConnectionGroup group = CONN_MAP.get(namespace);
         if (group == null) {
             synchronized (CREATE_LOCK) {
-                if (CONN_MAP.get(namespace) == null) {
+                if ((group = CONN_MAP.get(namespace)) == null) {
                     group = new ConnectionGroup(namespace);
                     CONN_MAP.put(namespace, group);
                 }