Fix: action executor: double work list unlock on error

The action executor executes its queued work items without holding the
work list lock. This can result in a double-unlock when a fatal error
occurs during the processing of a work item.

A check for the "should_quit" flag is added before unlocking since this
is the only other reason for the thread to exit its loop.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I8d6b1a3511174abd08848ac3677cdf8a326fa8c5