Make an automation that uses a State Trigger to monitor the state of sensor.living_room_temperature. When the value becomes unavailable it turns off climate.furnace (which also serves to turn off the heater switch). Probably a good idea to also have it notify you that this incident occurred.
If you wish, the same automation can also be used to detect when the sensor’s state changes from unavailable to a valid value and then turn climate.furnace back on.
Let me know if you need help creating the automation.