Fix exception when receiving POISON_PILL

POISON_PILL is causing writing to closed client due to logic in code.

core-java-modules/core-java-nio-2/src/main/java/com/baeldung/selector/EchoServer.java

@@ -49,12 +49,12 @@ public class EchoServer {
         if (new String(buffer.array()).trim().equals(POISON_PILL)) {
             client.close();
             System.out.println("Not accepting client messages anymore");
-        }
-
+        } else {
             buffer.flip();
             client.write(buffer);
             buffer.clear();
         }
+    }
 
     private static void register(Selector selector, ServerSocketChannel serverSocket) throws IOException {
         SocketChannel client = serverSocket.accept();