use undelying height_to_hash to check main chain

chia/full_node/full_node.py

@@ -1546,7 +1546,7 @@ async def skip_blocks(
             # we have already validated this block once, no need to do it again.
             # however, if this block is not part of the main chain, we need to
             # update the fork context with its additions and removals
-            if blockchain.height_to_hash(block.height) == header_hash:
+            if self.blockchain.height_to_hash(block.height) == header_hash:
                 # we're on the main chain, just fast-forward the fork height
                 fork_info.reset(block.height, header_hash)
             else: