udf: fix uninit-value use in udf_get_fileshortad

Check for overflow when computing alen in udf_current_aext to mitigate
later uninit-value use in udf_get_fileshortad KMSAN bug[1].
After applying the patch reproducer did not trigger any issue[2].

[1] https://syzkaller.appspot.com/bug?extid=8901c4560b7ab5c2f9df
[2] https://syzkaller.appspot.com/x/log.txt?x=10242227980000



Reported-by:  <syzbot+8901c4560b7ab5c2f9df@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=8901c4560b7ab5c2f9df


Tested-by:  <syzbot+8901c4560b7ab5c2f9df@syzkaller.appspotmail.com>
Suggested-by: Jan Kara <jack@suse.com>
Signed-off-by: Gianfranco Trad <gianf.trad@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20240925074613.8475-3-gianf.trad@gmail.com