wallet2: clear m_additional_tx_keys during bg sync

(cherry picked from commit 4b00f47d43d784447904d777f220a525044a3374)

src/wallet/wallet2.cpp

@@ -4598,6 +4598,7 @@ void wallet2::clear_user_data()
   for (auto i = m_transfers.begin(); i != m_transfers.end(); ++i)
     i->m_frozen = false;
   m_tx_keys.clear();
+  m_additional_tx_keys.clear();
   m_tx_notes.clear();
   m_address_book.clear();
   m_subaddress_labels.clear();