Fix comment about cross-checking the varnullingrels
The nullingrels match checks are not limited to debugging builds.
Oversight in commit
867be9c07.
Author: Richard Guo
Reviewed-by: Alvaro Herrera, Tom Lane, Robert Haas
Discussion: https://postgr.es/m/CAMbWs4_SDsdYD7DdQw7RXc3jv3axbg+RGZ7aSi9GaqX=F8hNVw@mail.gmail.com