Fix crash when using -M with empty result

`ledger -f /dev/null reg -M test` causes a segmentation fault,
see [bug 730](http://bugs.ledger-cli.org/show_bug.cgi?id=730) and duplicates [1080](http://bugs.ledger-cli.org/show_bug.cgi?id=1080) and [1084](http://bugs.ledger-cli.org/show_bug.cgi?id=1084) for details.