Replace static buf with a stack-allocated one in ReadControlFile

It's only used very locally within the function.

Reviewed-by: Robert Haas
Discussion: https://www.postgresql.org/message-id/7f86e06a-98c5-4ce3-8ec9-3885c8de0358@iki.fi

src/backend/access/transam/xlog.c

@@ -4314,7 +4314,7 @@ ReadControlFile(void)
 {
 	pg_crc32c	crc;
 	int			fd;
-	static char wal_segsz_str[20];
+	char		wal_segsz_str[20];
 	int			r;
 
 	/*