pgsql: Remove redundant null pointer checks before PQclear and PQconnin

Remove redundant null pointer checks before PQclear and PQconninfoFree

These functions already had the free()-like behavior of handling null
pointers as a no-op.  But it wasn't documented, so add it explicitly
to the documentation, too.

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/5faef9d582012433db9ad05af27a77bd591508e1

Modified Files
--------------
contrib/dblink/dblink.c                  |  6 ++----
contrib/postgres_fdw/postgres_fdw.c      | 24 ++++++++----------------
doc/src/sgml/libpq.sgml                  |  5 +++++
src/bin/pg_basebackup/streamutil.c       |  6 ++----
src/bin/pg_dump/pg_dumpall.c             |  3 +--
src/bin/psql/command.c                   |  3 +--
src/bin/psql/common.c                    |  3 +--
src/bin/psql/describe.c                  |  3 +--
src/fe_utils/query_utils.c               |  3 +--
src/interfaces/ecpg/ecpglib/descriptor.c |  3 +--
src/interfaces/ecpg/ecpglib/execute.c    |  3 +--
src/interfaces/libpq/fe-connect.c        |  6 ++----
src/interfaces/libpq/fe-exec.c           |  9 +++------
13 files changed, 29 insertions(+), 48 deletions(-)