Fix assertion failure when Parallel Append is run serially.

Fix assertion failure when Parallel Append is run serially.

Parallel-aware plan nodes must be prepared to run without parallelism
if it's not possible at execution time for whatever reason.  Commit
ab72716778128fb63d54ac256adf7fe6820a1185, 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