Accept peers without DAO header

If the peer returns empty header list as the dao answer, it may be because the peer is not in Mainnet or the peer hasn't finished syncing yet.

pyethapp/eth_service.py

@@ -86,9 +86,11 @@ def run(self):
         try:
             dao_headers = self.deferred.get(block=True, timeout=self.request_timeout)
             log.debug("received DAO challenge answer", proto=self.proto, answer=dao_headers)
-            result = len(dao_headers) == 1 and \
+            # Accept peers without DAO header
+            result = len(dao_headers) == 0 or (
                     dao_headers[0].hash == self.config['DAO_FORK_BLKHASH'] and \
                     dao_headers[0].extra_data == self.config['DAO_FORK_BLKEXTRA']
+            )
             self.chainservice.on_dao_challenge_answer(self.proto, result)
         except gevent.Timeout:
             log.debug('challenge dao timed out', proto=self.proto)