xfs: check for valid indices in xfs_iext_get_ext and xfs_iext_idx_to_irec

Based on an earlier patch from Lachlan McIlroy.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Lachlan McIlroy <lmcilroy@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>