fs/ntfs: Correct next_attribute validation

fs/ntfs: Correct next_attribute validation

Improved ad-hoc fuzzing coverage revealed a possible access violation
around line 342 of grub-core/fs/ntfs.c when accessing the attr_cur
pointer due to possibility of moving pointer "next" beyond of the end of
the valid buffer inside next_attribute. Prevent this for cases where
full attribute validation is not performed (such as on attribute lists)
by performing a sanity check on the newly calculated next pointer.

Fixes: 06914b614 (fs/ntfs: Correct attribute vs attribute list validation)
Signed-off-by: Andrew Hamilton <adhamilt@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>