Fix cleaning up of the keepAliveRunnable (#381)

(cherry picked from commit b159de8)

src/main/java/net/dv8tion/jda/core/audio/AudioWebSocket.java

@@ -35,10 +35,7 @@
 import java.util.Arrays;
 import java.util.List;
 import java.util.Map;
-import java.util.concurrent.RejectedExecutionException;
-import java.util.concurrent.ScheduledThreadPoolExecutor;
-import java.util.concurrent.ThreadFactory;
-import java.util.concurrent.TimeUnit;
+import java.util.concurrent.*;
 import java.util.concurrent.atomic.AtomicInteger;
 
 public class AudioWebSocket extends WebSocketAdapter
@@ -64,8 +61,8 @@ public class AudioWebSocket extends WebSocketAdapter
     private final String token;
     private boolean connected = false;
     private boolean ready = false;
-    private boolean shutdown;
-    private Runnable keepAliveRunnable;
+    private boolean shutdown = false;
+    private Future<?> keepAliveHandle;
     private String wssEndpoint;
     private boolean shouldReconnect;
 
@@ -223,7 +220,6 @@ public void onTextMessage(WebSocket websocket, String message)
         }
     }
 
-
     @Override
     public void onDisconnected(WebSocket websocket, WebSocketFrame serverCloseFrame, WebSocketFrame clientCloseFrame, boolean closedByServer)
     {
@@ -331,10 +327,10 @@ public void close(ConnectionStatus closeStatus)
                 );
             api.getClient().send(obj.toString());
         }
-        if (keepAliveRunnable != null)
+        if (keepAliveHandle != null)
         {
-            keepAlivePool.remove(keepAliveRunnable);
-            keepAliveRunnable = null;
+            keepAliveHandle.cancel(false);
+            keepAliveHandle = null;
         }
 
         if (audioConnection != null)
@@ -476,10 +472,10 @@ private InetSocketAddress handleUdpDiscovery(InetSocketAddress address, int ssrc
 
     private void setupKeepAlive(final int keepAliveInterval)
     {
-        if (keepAliveRunnable != null)
+        if (keepAliveHandle != null)
             LOG.fatal("Setting up a KeepAlive runnable while the previous one seems to still be active!!");
 
-        keepAliveRunnable = () ->
+        Runnable keepAliveRunnable = () ->
         {
             if (socket.isOpen() && socket.isOpen() && !udpSocket.isClosed())
             {
@@ -514,7 +510,7 @@ private void setupKeepAlive(final int keepAliveInterval)
 
         try
         {
-            keepAlivePool.scheduleAtFixedRate(keepAliveRunnable, 0, keepAliveInterval, TimeUnit.MILLISECONDS);
+            keepAliveHandle = keepAlivePool.scheduleAtFixedRate(keepAliveRunnable, 0, keepAliveInterval, TimeUnit.MILLISECONDS);
         }
         catch (RejectedExecutionException ignored) {} //ignored because this is probably caused due to a race condition
                                                       // related to the threadpool shutdown.