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.

src/buffer.c

@@ -959,11 +959,8 @@ archive_is_dev (void)
 static void
 short_read (idx_t status)
 {
-  idx_t left;			/* bytes left */
-  char *more;                   /* pointer to next byte to read */
-
-  more = record_start->buffer + status;
-  left = record_size - status;
+  idx_t left = record_size - status;		/* bytes left to read */
+  char *more = (char *) record_start + status;	/* address of next read */
 
   if (left && left % BLOCKSIZE == 0
       && (warning_option & WARN_RECORD_SIZE)