Made the thread destruction a little more robust.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402840

dlib/threads/threads_kernel_shared.cpp

@@ -107,6 +107,7 @@ namespace dlib
                 // in this case we just let the thread pool know that it
                 // should self destruct whenever it gets a chance
                 should_destruct = true;
+                data_ready.broadcast();
                 data_mutex.unlock();
             }
         }