exfat: fix out-of-bounds access of directory entries

In the Linux kernel, the following vulnerability has been resolved: exfat: fix out-of-bounds access of directory entries In the case of the directory size is greater than or equal to the cluster size, if start_clu becomes an EOF cluster(an invalid cluster) due to file system corruption, then the directory entry where ei->hint_femp.eidx hint is outside the directory, resulting in an out-of-bounds access, which may cause further file system corruption. This commit adds a check for start_clu, if it is an invalid cluster, the file or directory will be treated as empty.