Correct zpool list displaying invalid EXPANDSZ for unavailable pool vdevs

When pools are unavailable their vdevs are also unavailable which means
that vdev_max_asize remains at the default zero. This default was being
used to calculate vs_esize resulting in a negative number as vdev_asize >
vdev_max_asize, which caused zpool list -v to display 16.0E for EXPANDSZ
of these vdevs.