Fix: event-notifier: Groups may not have an error counter

Issue
=====
If no error counter is attached to an event notifier group, calling
the `record_error()` function leads to a NULL pointer dereference.

Solution
========
Check if the error_counter field is allocated before dereferencing it.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I124519d37efe8eb20296cfd74027642c8e8162a5