Fixed a race condition in the logger object. It was missing a needed call

to unregister_thread_end_handler().  What could happen in some scenarios is,
during program termination, a global part of the logger object could be destructed
when it still had outstanding thread end handlers registered to it.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402858

dlib/logger/logger_kernel_1.cpp

@@ -96,6 +96,7 @@ namespace dlib
     ~global_data (
     )
     {
+        unregister_thread_end_handler(*this,&global_data::thread_end_handler);
     }
 
 // ----------------------------------------------------------------------------------------