MFV/ZoL: Fix wrong assertion in libzfs diff error handling

In compare(), all error cases set the error code to EPIPE, so when an
error is set, the correct assertion to make is that the error is EPIPE,
not EINVAL.

Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@freqlabs.com>
Closes #8743
zfsonlinux/zfs@9dc41a769df164875d974c2431b2453e70e16c41

Submitted by: Ryan Moeller <ryan@freqlabs.com>
MFC after: 1 week
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D20118