Fix crash due to using an attribute from a collection that went out of scope

Summary: Probably a consequence of the const/non-const changes (detaching?)

Test Plan:
Untested, but based on a crash report by Allen, pointing to
ETMCalendar::alarms() calling BlockAlarmsAttribute::isAlarmTypeBlocked
on line 579.

Reviewers: dvratil, winterz

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D20079