open_source / mqtt-cpp

promise is not working because it is reset to null in the connectFailure callback. removing the set_value() fixed the issue of subscription after promise.

Test:
start mosquitto broker and the publisher test on a remote desktop.
disable the internet connection on the subscribing desktop.
start the subscriber test on the subscribing desktop.

at this point, the mqtt client keeps returning to the connectFailure callback

enable the internet connection again.

at this point, the mqtt client returns to the connectSuccess callback. subscriptions are activated and data is coming in.