extmod/vfs: Fix buffer overflow of string comparison in umount.

The comparison between the given unmount string and existing mount strings
were made by the given string, which leads to buffer overflow.

Fixes issue #13006.

Signed-off-by: Junwha <qbit@unist.ac.kr>

extmod/vfs.c

@@ -273,7 +273,7 @@ mp_obj_t mp_vfs_umount(mp_obj_t mnt_in) {
         mnt_str = mp_obj_str_get_data(mnt_in, &mnt_len);
     }
     for (mp_vfs_mount_t **vfsp = &MP_STATE_VM(vfs_mount_table); *vfsp != NULL; vfsp = &(*vfsp)->next) {
-        if ((mnt_str != NULL && !memcmp(mnt_str, (*vfsp)->str, mnt_len + 1)) || (*vfsp)->obj == mnt_in) {
+        if ((mnt_str != NULL && mnt_len == (*vfsp)->len && !memcmp(mnt_str, (*vfsp)->str, mnt_len)) || (*vfsp)->obj == mnt_in) {
             vfs = *vfsp;
             *vfsp = (*vfsp)->next;
             break;