Fix tiny memory leaks

Fix tiny memory leaks

Both check_application_name() and check_cluster_name() use
pg_clean_ascii() but didn't release the memory.  Depending on when the
GUC is set, this might be cleaned up at some later time or it would
leak postmaster memory once.  In any case, it seems better not to have
to rely on such analysis and make the code locally robust.  Also, this
makes Valgrind happier.

Author: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Jacob Champion <jchampion@timescale.com>
Discussion: https://www.postgresql.org/message-id/CAD21AoBmFNy9MPfA0UUbMubQqH3AaK5U3mrv6pSeWrwCk3LJ8g@mail.gmail.com