cryptonote: rework block blob size sanity check

Use the actual block weight limit, assuming that weight is always
greater or equal to size

src/cryptonote_basic/cryptonote_basic.h

@@ -422,6 +422,8 @@ namespace cryptonote
       FIELDS(*static_cast<block_header *>(this))
       FIELD(miner_tx)
       FIELD(tx_hashes)
+      if (tx_hashes.size() > CRYPTONOTE_MAX_TX_PER_BLOCK)
+        return false;
     END_SERIALIZE()
   };