Prevent a race between completing jobs on a user-exclusive node from leaving the node owned.

Two jobs completing simultaneously leads to make_node_idle()
returning before it has a chance to decrement node_ptr->owner_job_cnt,
which can result in the node being "owned" by that user even
through no jobs are running on it.

Move the decrement block to the end at a fini label, and make sure
all return paths pass through it. While moving that add a guard
against node_ptr->owner_job_cnt underflowing.

Bug 3771.