Parallel-aware plan nodes must be prepared to run without parallelism
if it's not possible at execution time for whatever reason.  Commit
ab727167, which introduced Parallel
Append, overlooked this.

Rajkumar Raghuwanshi reported this problem, and I included his test
case in this patch.  The code changes are by me.

Discussion: http://postgr.es/m/CAKcux6=WqkUudLg1GLZZ7fc5ScWC1+Y9qD=pAHeqy32WoeJQvw@mail.gmail.com