Make sure exc_obj is always defined

Otherwise, it ends up being loaded using `LOAD_FAST_CHECK`, which increfs
and causes the refcount check to fail when it uses `LOAD_FAST_BORROW`.

Author: mpage

Lib/test/test_traceback.py

@@ -3598,6 +3598,7 @@ def test_no_save_exc_type(self):
             self.assertIsNone(te.exc_type)
 
     def test_no_refs_to_exception_and_traceback_objects(self):
+        exc_obj = None
         try:
             1/0
         except Exception as e: