C-c C-c ledger-toggle-current searches backwards indefinitely

[jstamant]

If the cursor isn't on a transaction, hitting C-c C-c (ledger-toggle-current) will search backwards until it finds a transaction, and then toggles its state.

The behavior is the same as in issue #316, where I explain this problem in a bit more detail. Like in #316, my suggestion here is to add some validation that only lets a user perform ledger-toggle-current when point is on a valid transaction or posting.

I'll probably provide a fix in a PR shortly.