Fix wrong assertion in libzfs diff error handling

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