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

sys/dev/random/fortuna.c

@@ -310,6 +310,8 @@ random_fortuna_genblocks(uint8_t *buf, u_int blockcount)
 	u_int i;
 
 	RANDOM_RESEED_ASSERT_LOCK_OWNED();
+	KASSERT(!uint128_is_zero(fortuna_state.fs_counter), ("FS&K: C != 0"));
+
 	for (i = 0; i < blockcount; i++) {
 		/*-
 		 * FS&K - r = r|E(K,C)