pgsql: In XLogFileInit(), fix *use_existent postcondition to suit calle

In XLogFileInit(), fix *use_existent postcondition to suit callers.

Infrequently, the mismatch caused log_checkpoints messages and
TRACE_POSTGRESQL_CHECKPOINT_DONE() to witness an "added" count too high
by one.  Since that consequence is so minor, no back-patch.

Discussion: https://postgr.es/m/20210202151416.GB3304930@rfd.leadboat.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/85656bc3050f0846f53de95768b0f9f9df410560

Modified Files
--------------
src/backend/access/transam/xlog.c | 13 ++++++-------
1 file changed, 6 insertions(+), 7 deletions(-)