Fix segfault in zpool iostat when adding VDEVs
Fix a segfault when running 'zpool iostat -v 1' while adding
a VDEV.
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #6748
Closes #6872