MQTT固定头剩余长度（可变长度）

remain length为可变长整数，最少一个字节，最多4个字节

每字节最高位作为后续字节标记位，剩余7字节作为字节长度

uint32_t MQTT_variable_length_decode(uint8_t *code)
{
    uint8_t temp = 0;
    uint8_t cnt = 0;
    uint32_t length = 0;
    long multiplier = 1;
   do {
        length +=  (code[cnt] & 0x7f) * multiplier;
           multiplier *= 128;
   }while((code[cnt] & 0x80) != 0)
}

uint8_t  MQTT_variable_length_encode(uint8_t *code, uint32_t length)
{
    long divider = 1;
    uint8_t cnt = 0;
    do {
        code[cnt] = length % 128u;
        length /= 128u;
        if (length > 0)
            code[cnt] |= 0x80;
    }while(length > 0)
}