carrot_impl: fix background sync key images

src/wallet/wallet2.cpp

@@ -2368,6 +2368,9 @@ void wallet2::scan_key_image(const wallet::enote_view_incoming_scan_info_t &enot
 {
   ki_out = std::nullopt;
 
+  if (m_multisig || m_background_syncing) // no spend privkey
+    return;
+
   // if keys are encrypted, ask for password
   if (is_spendkey_encryption_enabled())
   {