index: don't include nullid in boundary check in pure code

Yet another change that should probably have been part of a3dacabd476b
(index: don't allow index[len(index)] to mean nullid, 2018-07-20).

Differential Revision: https://phab.mercurial-scm.org/D4333

mercurial/pure/parsers.py

@@ -47,7 +47,7 b' class BaseIndexObject(object):'
     def _fix_index(self, i):
         if not isinstance(i, int):
             raise TypeError("expecting int indexes")
-        if i < 0 or i >= len(self) + 1:
+        if i < 0 or i >= len(self):
             raise IndexError
         return i