pgsql: Remove redundant null pointer checks before pg_free()

Remove redundant null pointer checks before pg_free()

These are especially useless because the whole point of pg_free() was
to do that very check before calling free().

pg_free() could be removed altogether, but I'm keeping it here to keep
the API consistent.

Discussion: https://www.postgresql.org/message-id/flat/dac5d2d0-98f5-94d9-8e69-46da2413593d%40enterprisedb.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/098c703d308fa88dc9e3f9f623ca023ce4717794

Modified Files
--------------
src/bin/pg_basebackup/walmethods.c |  3 +--
src/bin/pg_upgrade/parallel.c      | 18 ++++++------------
src/bin/pgbench/pgbench.c          |  9 +++------
src/bin/psql/command.c             |  3 +--
src/bin/psql/variables.c           |  5 ++---
5 files changed, 13 insertions(+), 25 deletions(-)