Add assertion to dsl_dataset_set_compression_sync

Coverity pointed out that if we somehow receive SPA_FEATURE_NONE, we
will use a negative number as an array index. A defensive assertion
seems appropriate.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Neal Gompa <ngompa@datto.com>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13872