sanitize pass_checkpoint

lib/blockchain.py

@@ -55,9 +55,17 @@ class Blockchain(util.PrintError):
         t.start()
 
     def pass_checkpoint(self, header):
+        if type(header) is not dict:
+            return False
         if header.get('block_height') != self.checkpoint_height:
             return True
-        return self.hash_header(header) == self.checkpoint_hash
+        if header.get('prev_block_hash') is None:
+            header['prev_block_hash'] = '00'*32
+        try:
+            _hash = self.hash_header(header)
+        except:
+            return False
+        return _hash == self.checkpoint_hash
 
     def verify_header(self, header, prev_header, bits, target):
         prev_hash = self.hash_header(prev_header)