wallet2: fix the logged number of detached transfers

src/wallet/wallet2.cpp

@@ -3480,6 +3480,7 @@ void wallet2::detach_blockchain(uint64_t height, std::map<std::pair<uint64_t, ui
     THROW_WALLET_EXCEPTION_IF(it_pk == m_pub_keys.end(), error::wallet_internal_error, "public key not found");
     m_pub_keys.erase(it_pk);
   }
+  transfers_detached = std::distance(it, m_transfers.end());
   m_transfers.erase(it, m_transfers.end());
 
   size_t blocks_detached = m_blockchain.size() - height;