The previous code failed to realize that this setting effectively
disables parallelism, and would crash if it decided to attempt
parallelism anyway.  Instead, treat it as a disabling condition.

Kyotaro Horiguchi, who also reported the issue. Reviewed by Michael
Paquier and Peter Geoghegan.

Discussion: http://postgr.es/m/20180209.170635.256350357.horiguchi.kyotaro@lab.ntt.co.jp