Fix off-by-one in XLogRecordMaxSize check.

Fix off-by-one in XLogRecordMaxSize check.

pg_logical_emit_message(false, '_', repeat('x', 1069547465)) failed with
self-contradictory message "WAL record would be 1069547520 bytes (of
maximum 1069547520 bytes)".  There's no particular benefit from allowing
or denying one byte in either direction; XLogRecordMaxSize could rise a
few megabytes without trouble.  Hence, this is just for cleanliness.
Back-patch to v16, where this check first appeared.