Small code simplification
Apply the same code simplification to ATExecAddColumn as was done in
7ff9afbbd: apply GETSTRUCT() once instead of doing it repeatedly in
the same function.
Author: Tender Wang
Discussion: https://postgr.es/m/CAHewXNkO9+U437jvKT14s0MCu6Qpf6G-p2mZK5J9mAi4cHDgpQ@mail.gmail.com