Problem with negative temperature / signed values

[lobl]

[x] I've read the Troubleshooting Wiki, my problem is not described there and I am already using the specified minimum bluez version.

Describe the bug

Put a lywsd03mmc sensor into the freezer, which is around -18 to -25 C.
The sensor works fine for months now and shows the correct temperature on the internal display.
But on MQTT the temperature shows up false, it's always in the area of 6532.2 C, depending on the freezers temperature. Looks like a problem with signed values. Or can it be configured somewhere?

To Reproduce
Steps to reproduce the behavior:

1. Put sensor in the freezer
2. Edit config.yml (see config section)
3. Start docker container
4. Check MQTT for the temperature

Expected behavior
Negative temperature from the sensor in the freezer should appear in MQTT, instead a positive temperature shows up.

Config

config.yml:

mqtt:
  host: 192.168.178.109
  port: 1883
  username:
  password:
  client_id: bt-mqtt-gateway
  availability_topic: #status/bluetooth2mqtt

manager:
  workers:
    lywsd03mmc:
      args:
        devices:
          gefrierfach:  a4:c1:38:4f:1d:f0
        topic_prefix: tele
        passive: true         # Set to true for sensors running custom firmware and advertising type custom. See https://github.com/zewelor/bt-mqtt-gateway/wiki/Devices#lywsd03mmc
        #command_timeout: 30   # Optional timeout for getting data for non-passive readouts
        scan_timeout: 20      # Optional timeout for passive scanning
      update_interval: 30

Debug gateway logs

There is no error in the log. It seems fine, except the temperature is way off.

2022-02-05 17:56:23,349 DEBUG bt-mqtt-gw.workers_manager workers_manager.py:57:execute - Execution result of command Lywsd03MmcWorker.status_update:
[
{
"topic": "tele/gefrierfach",
"payload": "{"temperature": 6532.3, "humidity": 71, "battery": 26}"
}
]

Server (please complete the following information):

• OS: Linux
• Distro: Raspbian
• Version 10
• "Raspbian GNU/Linux 10 (buster)"
• Installed packages are on the latest version. bluez is on V. 5.50

Additional context

Information from sensor (via TelinkMiFlasher):

Advertising type: Atc1441
Hardware Version: LYWSD03MMC B1.4, Software Version: 3.5, Sensor: SHT4x

[jewelooper]

The processScanValue method is missing a conversion to a signed integer value.
Something like this should supply correct positive and negative temperature values:

temperature = int(data[16:20], 16)
temperature = ((temperature+0x8000)&0xFFFF) - 0x8000
temperature = temperature / 10