[emacs] Make ledger-occur-mode auto-disable when restored by desktop.el

The minor mode gets restored by desktop.el, and this has long caused
errors. While the ideal would be to restore the mode along with the
current narrowing regexp, this interim fix instead ensures that
ledger-occur-mode will disable itself if it finds no regexp set.
Fix crash when using -M with empty result

ledger -f /dev/null reg -M test causes a segmentation fault,
see bug 730 and duplicates 1080 and 1084 for details.

Kudos to Ikke for helping with debugging.
Merge pull request #340 from afh/pull/730

Fix crash when using -M with empty result