Fortuna: Add trivial assert to match FS&K definition

FS&K GenerateBlocks function asserts C (counter) != 0. This should also
be true in our implementation.

Reviewed by: delphij, markm
Approved by: secteam (delphij)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D16936
(cherry picked from commit 84880efae7040bb55e6f63166f53158135574e3a)