Remove redundant memset call following palloc0

Remove redundant memset call following palloc0

This is a follow-up commit to ca7f8e2 which removed the allocation
abstraction from pgcrypto and replaced px_alloc + memset calls with
palloc0 calls. The particular memset in this commit was missed in
that work though.

Author: Zhihong Yu <zyu@yugabyte.com>
Reviewed-by: Bruce Momjian <bruce@momjian.us>
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://postgr.es/m/CALNJ-vT5qRucrFMPSzQyAWods1b4MnNPG-M=_ZUzh1SoTh0vNw@mail.gmail.com