Fix VD-1301 and VD-1302 vulnerabilities

This patch was contributed by Maor Vermucht and Or Peles from
VDOO Connected Trust.
int modbus_receive_confirmation(modbus_t *ctx, uint8_t *rsp)
{
    if (ctx == NULL) {
        errno = EINVAL;
        return -1;
    }

    return _modbus_receive_msg(ctx, rsp, MSG_CONFIRMATION);
}

int modbus_receive_confirmation(modbus_t *ctx, uint8_t *rsp)
{
    if (ctx == NULL) {
        errno = EINVAL;
        return -1;
    }

    return _modbus_receive_msg(ctx, rsp, MSG_CONFIRMATION);
}
