Fix an issue that enableConcurrent may be set to true after Inbox is stopped

Because `Inbox.stop()` may run before processing `OnStart`, it's possible that `enableConcurrent` will be set to `true` after Inbox is stopped in the previous codes. This is not a correct behavior.

This commit just added a check before setting `enableConcurrent` to `true` to fix this issue.

Author: zsxwing

core/src/main/scala/org/apache/spark/rpc/netty/Inbox.scala

@@ -129,7 +129,9 @@ private[netty] class Inbox(
             endpoint.onStart()
             if (!endpoint.isInstanceOf[ThreadSafeRpcEndpoint]) {
               inbox.synchronized {
-                enableConcurrent = true
+                if (!stopped) {
+                  enableConcurrent = true
+                }
               }
             }
           }