_MessageBase should assert packet size

[JPHutchins]

When forming packets, the head provides a length field. Before the packet is formed, it should be asserted that the length field does indeed match the length of the CBOR payload.

Zephyr asserts this in smp.c

        /* Read the management header and strip it from the request. */
        rc = smp_read_hdr(req, &req_hdr);
        if (rc != 0) {
            rc = MGMT_ERR_ECORRUPT;
            break;
        } else {
            valid_hdr = true;
        }
        /* Does buffer contain whole message? */
        if (req->len < (req_hdr.nh_len + MGMT_HDR_SIZE)) {
            rc = MGMT_ERR_ECORRUPT;
            break;
        }