fs/hfsplus: Set grub_errno to prevent NULL pointer access

fs/hfsplus: Set grub_errno to prevent NULL pointer access

When an invalid node size is detected in grub_hfsplus_mount(), data
pointer is freed. Thus, file->data is not set. The code should also
set the grub_errno when that happens to indicate an error and to avoid
accessing the uninitialized file->data in grub_file_close().

Signed-off-by: Lidong Chen <lidong.chen@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>