Fix: ust-app: error counter index never set

Issue
=====
The `struct ust_app_event_notifier_rule::error_counter_index` field is
currently never set so all error counting is done in bucket index 0.

Fix
===
Set the error counter index.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I951095dee5586111699b0ea0e3bc26c0fb4a535d