hfsplus: Fix two more overflows
    
    Both node->size and node->namelen come from the supplied filesystem,
    which may be user-supplied. We can't trust them for the math unless we
    know they don't overflow. Making sure they go through grub_add() or
    grub_calloc() first will give us that.
    
    Signed-off-by: Peter Jones <pjones@redhat.com>
    Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
    Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>