Fix ASSERT in zfs_receive_one()

Fix ASSERT in zfs_receive_one()

This commit fixes the following ASSERT in zfs_receive_one() when
receiving a send stream from a root dataset with the "-e" option:

    $ sudo zfs snap source@snap
    $ sudo zfs send source@snap | sudo zfs recv -e destination/recv
    chopprefix > drrb->drr_toname
    ASSERT at libzfs_sendrecv.c:3804:zfs_receive_one()

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #8121