Port short_read to UBSan

Port short_read to UBSan

Problem reported by Kirill Furman in:
https://lists.gnu.org/r/bug-tar/2025-06/msg00002.html
* src/buffer.c (short_read): Use (char *) record_start,
instead of record_start->buffer, to avoid undefined behavior
accessing past end of buffer.  In practice the undefined
behavior is harmless unless running with -fsanitize=undefined
or a similarly-picky implementation.