Fix lazy loaded sync with rejected state events

In particular, we assume that the name and canonical alias events in
the state have not been rejected. In practice this may not be the case
(though we should probably think about fixing that) so lets ensure that
we gracefully handle that case, rather than 404'ing the sync request
like we do now.

synapse/handlers/sync.py

@@ -567,13 +567,13 @@ class SyncHandler(object):
         # be a valid name or canonical_alias - i.e. we're checking that they
         # haven't been "deleted" by blatting {} over the top.
         if name_id:
-            name = yield self.store.get_event(name_id, allow_none=False)
+            name = yield self.store.get_event(name_id, allow_none=True)
             if name and name.content:
                 defer.returnValue(summary)
 
         if canonical_alias_id:
             canonical_alias = yield self.store.get_event(
-                canonical_alias_id, allow_none=False,
+                canonical_alias_id, allow_none=True,
             )
             if canonical_alias and canonical_alias.content:
                 defer.returnValue(summary)