Fix Parallel Append crash.

Reported by Tom Lane and the buildfarm.

Amul Sul and Amit Khandekar

Discussion: http://postgr.es/m/17868.1512519318@sss.pgh.pa.us
Discussion: http://postgr.es/m/CAJ3gD9cJQ4d-XhmZ6BqM9rMM2KDBfpkdgOAb4+psz56uBuMQ_A@mail.gmail.com
