Fix a bug during initial blockchain check

An invalid block in the locally stored blockchain caused
the node to crash.

blockchain.py

@@ -186,7 +186,7 @@ class Blockchain:
         self.__lock = Lock()
         self.__load_blocks_from_disk()
     def __load_blocks_from_disk(self):
-        block_obj = None
+        last_valid = None
         try:
             with open("blockchain", "rb") as f:
                 while True:
@@ -196,10 +196,11 @@ class Blockchain:
                     block_obj = self.add_block(block)
                     if not block_obj.validate(self):
                         break
+                    last_valid = block_obj
         except FileNotFoundError:
             pass
-        if block_obj is not None:
-            self.set_latest_block(block_obj.own_hash)
+        if last_valid is not None:
+            self.set_latest_block(last_valid.own_hash)
     def set_latest_block(self, block_hash):
         new_block = self.get_block(block_hash)
         assert new_block is not None