Fixed|libcore|TaskPool: Completing a pool while it is being destroyed

The allTasksDone() signal was emitted after unlocking, which meant that
a destructor waiting on the mutex would get a chance to complete too
early.

doomsday/libcore/src/concurrency/taskpool.cpp

@@ -102,14 +102,10 @@ DENG2_PIMPL(TaskPool), public Lockable, public Waitable, public TaskPool::IPool
             }
             else
             {
-                unlock();
                 emit self.allTasksDone();
             }
         }
-        else
-        {
-            unlock();
-        }
+        unlock();
     }
 };