fix bug if headers file is empty

lib/blockchain.py

@@ -286,6 +286,8 @@ class Blockchain(util.PrintError):
         height = header['block_height']
         if self.height() != height - 1:
             return False
+        if height == 0:
+            return hash_header(header) == bitcoin.GENESIS
         previous_header = self.read_header(height -1)
         if not previous_header:
             return False

lib/network.py

@@ -990,11 +990,19 @@ class Network(util.DaemonThread):
             self.notify('interfaces')
             self.switch_lagging_interface()
             return
+        tip = max([x.height() for x in self.blockchains.values()])
+        if tip >=0:
             interface.mode = 'backward'
             interface.bad = height
             interface.bad_header = header
-        tip = max([x.height() for x in self.blockchains.values()])
             self.request_header(interface, tip)
+        else:
+            chain = self.blockchains[0]
+            if chain.catch_up is None:
+                chain.catch_up = interface
+                interface.mode = 'catch_up'
+                interface.blockchain = chain
+                self.request_header(interface, 0)
 
     def blockchain(self):
         if self.interface and self.interface.blockchain is not None: