sshbuf_poke_u8
ASSERT_INT_EQ(sshbuf_poke_u8(p1, 0, 0xa1), 0);
ASSERT_INT_EQ(sshbuf_poke_u8(p1, 9, 0xa1), 0);
ASSERT_INT_EQ(sshbuf_poke_u8(p1, 10, 0xa1), SSH_ERR_NO_BUFFER_SPACE);
ASSERT_INT_EQ(sshbuf_poke_u8(p1, 1000, 0xa1), SSH_ERR_NO_BUFFER_SPACE);
int sshbuf_poke_u8(struct sshbuf *buf, size_t offset, u_char val);