Added missing mutex unlock statement.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403439

dlib/threads/threads_kernel_shared.cpp

@@ -109,6 +109,13 @@ namespace dlib
                 data_mutex.unlock();
                 delete this;
             }
+            else
+            {
+                // There are still some user threads running so there isn't
+                // much we can really do.  Just let the program end without
+                // cleaning up threading resources.  
+                data_mutex.unlock();
+            }
         }
 
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