[nemo-qml-plugin-calendar] Fix setting invalid end date to recurrence

KCalCore gets confused when setting invalid end date. On such, there
remains occurrences infinitely, but Recurrence::recursAt(),
getNextDateTime() don't work properly anymore.
Merge pull request #45 from pvuorela/recur_end_date

[nemo-qml-plugin-calendar] Fix setting invalid end date to recurrence