Fix NULL pointer dereference in 'zfs create'

Fix NULL pointer dereference in 'zfs create'

A race condition between 'zpool export' and 'zfs create' can crash the
latter: this is because we never check libzfs`zpool_open() return
value in libzfs`zfs_create().

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6096