Added missing mutex lock

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402691

dlib/threads/thread_pool_extension.cpp

@@ -46,6 +46,7 @@ namespace dlib
     num_threads_in_pool (
     ) const
     {
+        auto_mutex M(m);
         return tasks.size();
     }