backend tests progress

backends/postgres/src/postgres_connection.cpp

@@ -25,9 +25,10 @@ void postgres_connection::open()
 
   conn_ = PQconnectdb(connection.c_str());
   if (PQstatus(conn_) == CONNECTION_BAD) {
-    const auto msg = PQerrorMessage(conn_);
+    const std::string msg = PQerrorMessage(conn_);
     PQfinish(conn_);
-    throw_postgres_error(msg, "postgres");
+    conn_ = nullptr;
+    throw_postgres_error(msg.c_str(), "postgres");
   }
 }