store: assert the fncache have been loaded if dirty

This should catch fncache corruption as the one that existed in
`perffncachewrite`.

mercurial/store.py

@@ -449,6 +449,7 b' class fncache(object):'
 
     def write(self, tr):
         if self._dirty:
+            assert self.entries is not None
             tr.addbackup('fncache')
             fp = self.vfs('fncache', mode='wb', atomictemp=True)
             if self.entries: