fix deadlock at exit

direct/src/showbase/Loader.py

@@ -52,7 +52,7 @@ class Loader(DirectObject):
     def destroy(self):
         self.ignore(self.hook)
         # Temp condition for old Panda.
-        if hasattr(loader, "stopThreads"):
+        if hasattr(self.loader, "stopThreads"):
             self.loader.stopThreads()
         del self.base
         del self.loader