network: fix fork detection in certain backward search cases

if there are no checkpoints and the split is very old, the exponential backward search might undershoot so that it asks for the genesis block but 0 and None are not distinguished hence the fork detection logic fails

lib/network.py

@@ -974,7 +974,7 @@ class Network(util.DaemonThread):
         else:
             raise Exception(interface.mode)
         # If not finished, get the next header
-        if next_height:
+        if next_height is not None:
             if interface.mode == 'catch_up' and interface.tip > next_height + 50:
                 self.request_chunk(interface, next_height // 2016)
             else: