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

src/backend/access/transam/xlog.c

@@ -3261,8 +3261,8 @@ XLogNeedsFlush(XLogRecPtr record)
  * logsegno: identify segment to be created/opened.
  *
  * *use_existent: if true, OK to use a pre-existing file (else, any
- * pre-existing file will be deleted).  On return, true if a pre-existing
- * file was used.
+ * pre-existing file will be deleted).  On return, false iff this call added
+ * some segment on disk.
  *
  * Returns FD of opened file.
  *
@@ -3431,8 +3431,10 @@ XLogFileInit(XLogSegNo logsegno, bool *use_existent)
 	 * CheckPointSegments.
 	 */
 	max_segno = logsegno + CheckPointSegments;
-	if (!InstallXLogFileSegment(&installed_segno, tmppath,
-								*use_existent, max_segno))
+	if (InstallXLogFileSegment(&installed_segno, tmppath,
+							   *use_existent, max_segno))
+		*use_existent = false;
+	else
 	{
 		/*
 		 * No need for any more future segments, or InstallXLogFileSegment()
@@ -3442,9 +3444,6 @@ XLogFileInit(XLogSegNo logsegno, bool *use_existent)
 		unlink(tmppath);
 	}
 
-	/* Set flag to tell caller there was no existent file */
-	*use_existent = false;
-
 	/* Now open original target segment (might not be file I just made) */
 	fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method));
 	if (fd < 0)