Fix two abd_gang_add_gang() issues.

• There is no reason to assert that added gang is not empty. It

may be weird to add an empty gang, but it is legal.

• When moving chain list from the added gang clear its size, or it

will trigger assertion in abd_verify() when that gang is freed.

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #14816