Fixing a potential segfault during drawing

This patch moves a lock inside the thread finishing
code such that a race condition of the finish sequence
and a random restart of the threads becomes impossible.
This problem has been observed while randomly zooming
with multiple drawing threads present. It's hard to
reproduce.

src/tl/tl/tlThreadedWorkers.cc

@@ -412,9 +412,9 @@ JobBase::stop ()
   m_stopping = false;
   m_running = false;
 
-  m_lock.unlock ();
-
   stopped ();
+
+  m_lock.unlock ();
 }
 
 void