SW freezes after several hours if MQTT option is activated

[javiser]

When the MQTT option is activated (#define MQTT_ACTIVE is set in src/mqtt_config.hpp) the SW freezes after several hours. The MQTT functionality is working. When the option is not set, the SW remains stable for days.
I have tried already:

• To make sure that the MQTT connection is done only once (see commit 66106d9)
• Set some dummy password so that the connection with the MQTT broker wouldn't work
  In these cases the problem persists.
  As workaround I will turn off the MQTT option as default until this is clarified