sshbuf_poke_u64
ASSERT_INT_EQ(sshbuf_poke_u64(p1, 0, 0xa1b2c3d4e5f60718ULL), 0);
ASSERT_INT_EQ(sshbuf_poke_u64(p1, 2, 0xa1b2c3d4e5f60718ULL), 0);
ASSERT_INT_EQ(sshbuf_poke_u64(p1, 3, 0xa1b2c3d4e5f60718ULL),
ASSERT_INT_EQ(sshbuf_poke_u64(p1, 10, 0xa1b2c3d4e5f60718ULL),
ASSERT_INT_EQ(sshbuf_poke_u64(p1, 1000, 0xa1b2c3d4e5f60718ULL),
int sshbuf_poke_u64(struct sshbuf *buf, size_t offset, uint64_t val);