random: Replace a comment with a static assertion
No functional change intended.
Reviewed by: cem
MFC after: 1 week
Sponsored by: Stormshield
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D51110
(cherry picked from commit 6ccf1801f225a5e3e71d5b707646731542a994c7)