only default to wrapped if instance is actually None

fixes #145

src/wrapt/decorators.py

@@ -496,7 +496,7 @@ def _synchronized_wrapper(wrapped, instance, args, kwargs):
         # desired context is held. If instance is None then the
         # wrapped function is used as the context.
 
-        with _synchronized_lock(instance or wrapped):
+        with _synchronized_lock(instance if instance is not None else wrapped):
             return wrapped(*args, **kwargs)
 
     class _FinalDecorator(FunctionWrapper):