Make use of pg_memory_is_all_zeros() in more places

Hi hackers,

While searching for memcmp() calls in "*stat*.c" files (due to [1]), it appeared
that we could $SUBJECT. Please find attached a patch doing so.

While at it, I did a quick check on all the memcmp() calls (even those not in
"*stat*.c" files) and it looks to me that there is no others that could be replaced
with pg_memory_is_all_zeros().

[1]: https://www.postgresql.org/message-id/flat/Z1hNLvcPgVLPxCoc%40ip-10-97-1-34.eu-west-3.compute.internal

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com