SPIGOT-5867, MC-193339: NPE during shutdown when rcon enabled with no password

nms-patches/RemoteControlListener.patch

@@ -0,0 +1,11 @@
+--- a/net/minecraft/server/RemoteControlListener.java
++++ b/net/minecraft/server/RemoteControlListener.java
+@@ -109,7 +109,7 @@
+         RemoteControlListener.LOGGER.debug("closeSocket: {}", serversocket);
+ 
+         try {
+-            serversocket.close();
++            if (serversocket != null) serversocket.close(); // CraftBukkit - SPIGOT-5867, MC-193339: NPE during shutdown if no password
+         } catch (IOException ioexception) {
+             RemoteControlListener.LOGGER.warn("Failed to close socket", ioexception);
+         }