db: throw when given a non txout_to_key output to add

The check was explicit in the original version, so it seems
safer to make it explicit here, especially as it is now done
implicitely in a different place, away from the original check.

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -695,6 +695,10 @@ void BlockchainLMDB::add_output(const crypto::hash& tx_hash, const tx_out& tx_ou
     if (mdb_put(*m_write_txn, m_output_keys, &k, &data, 0))
       throw0(DB_ERROR("Failed to add output pubkey to db transaction"));
   }
+  else
+  {
+    throw0(DB_ERROR("Wrong output type: expected txout_to_key"));
+  }
 
   m_num_outputs++;
 }