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.

This does solve the issue, yet my understanding of boost and the used deck here is too little to potentially see a better solution for the problem at hand.

Please review critically and scrutinisingly, thanks.