Fix postgres notification subscription on LISTEN failure

Register a notification only if the LISTEN call was successful. This
behaves now like the UNLISTEN case: the notification is only unregistered
if the call was successful.

Change-Id: I2b18ec3a619fea5bed0a319013ad9df4b9e15456
Reviewed-by: Matt Newell <newellm@blur.com>
Reviewed-by: Jesus Fernandez <Jesus.Fernandez@qt.io>
Reviewed-by: Andy Shaw <andy.shaw@qt.io>

src/plugins/sqldrivers/psql/qsql_psql.cpp

@@ -1420,6 +1420,7 @@ bool QPSQLDriver::subscribeToNotification(const QString &name)
         QString query = QLatin1String("LISTEN ") + escapeIdentifier(name, QSqlDriver::TableName);
         PGresult *result = d->exec(query);
         if (PQresultStatus(result) != PGRES_COMMAND_OK) {
+            d->seid.removeLast();
             setLastError(qMakeError(tr("Unable to subscribe"), QSqlError::StatementError, d, result));
             PQclear(result);
             return false;