Set thread cleanup handlers to null when deleting it so that calling on_thread_exit() twice doesn't use an invalid pointer.

[SVN r24228]

src/threadmon.cpp

@@ -117,7 +117,8 @@
             }
 
             //Destroy the exit handlers
-            delete handlers;
+            if (TlsSetValue(key, 0))
+                delete handlers;
         }
     }