fix: use double check to fix concurrent problem (#1909)

Co-authored-by: zhangwenhe <zhangwh@ehualu.com>

leader-followers/src/main/java/com/iluwatar/leaderfollowers/Worker.java

@@ -57,9 +57,11 @@ public class Worker implements Runnable {
       try {
         if (workCenter.getLeader() != null && !workCenter.getLeader().equals(this)) {
           synchronized (workCenter) {
-            workCenter.wait();
+            if (workCenter.getLeader() != null && !workCenter.getLeader().equals(this)) {
+              workCenter.wait();
+              continue;
+            }
           }
-          continue;
         }
         final Task task = taskSet.getTask();
         synchronized (workCenter) {