revlog: reset _nodepos after strip

When using the pure revlog parser, _nodepos is used to keep track of the
position during index scanning in the non-cached cache. If it is out of
bounds, BaseIndexObject._fix_index will assert. Since strip can actually
remove the position scanned last, make sure to reset it. Add an
assertion in the place where the invariance is clearer.

Differential Revision: https://phab.mercurial-scm.org/D3188

mercurial/revlog.py

@@ -818,6 +818,8 b' class revlog(object):'
             p = self._nodepos
             if p is None:
                 p = len(i) - 2
+            else:
+                assert p < len(i)
             for r in xrange(p, -1, -1):
                 v = i[r][7]
                 n[v] = r
@@ -2421,6 +2423,7 b' class revlog(object):'
             del self.nodemap[self.node(x)]
 
         del self.index[rev:-1]
+        self._nodepos = None
 
     def checksize(self):
         expected = 0