udf: fix uninit-value use in udf_get_fileshortad

In the Linux kernel, the following vulnerability has been resolved: 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