Don't set the root node before it's not actually defined

parso/pgen2/parse.py

@@ -146,7 +146,6 @@ class PgenParser(object):
         self.convert_leaf = convert_leaf
 
         self.stack = Stack([StackNode(grammar._nonterminal_to_dfas[start_nonterminal][0])])
-        self.rootnode = None
         self.error_recovery = error_recovery
 
     def parse(self, tokens):