Fix divide by zero during indirect split damage

This patch simply ensures that vdev_indirect_splits_damage()
cannot hit a divide by zero exception if a split has no
children with valid data. The normal reconstruction code
path in vdev_indirect_reconstruct_io_done() already has this
check.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8086