Don't attempt to retain the buffer if it goes to a closed connection

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientPlaySessionHandler.java

@@ -256,7 +256,7 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
     }
 
     MinecraftConnection smc = serverConnection.getConnection();
-    if (smc != null && serverConnection.getPhase().consideredComplete()) {
+    if (smc != null && !smc.isClosed() && serverConnection.getPhase().consideredComplete()) {
       smc.write(buf.retain());
     }
   }