[java] Fix locking bug in Java ThreadPool.

Fix a bug that made it possible that the tasks_lock could be locked but
never unlocked if an exception was thrown.

java/runtime/MercuryThreadPool.java:
    As above.

java/runtime/MercuryThreadPool.java

@@ -455,6 +455,7 @@ public class MercuryThreadPool
                  * Have all the tasks been completed?
                  */
                 tasks_lock.lock();
+                tasks_locked = true;
                 try {
                     num_tasks_submitted = this.num_tasks_submitted;
                     num_tasks_completed = this.num_tasks_completed;