ristsender/receiver: disable stats printing when interval is 0

ristsender/receiver option -S 0 was not disabling stats printing since the callback is always registered and the default stats report interval is not zero.

This PR stops the stats callback being registered is statsinterval is set to 0 on the command line. It may be that the prefered behaviour would be to change the stats interval inside the library, but I wasn't sure so went with the simple option. Please feel free to reject if you have a better solution!