librist does not recover from fifo queue overflow properly

If a fifo queue overflow happens, rist_receiver_data_read starts to read entires in the fifo queue with data_block == NULL and the code which is commented as "I think this should never happen" fires, rist_receiver_data_read returns 0 when in fact there are entries in the queue...

I have attached a prelimenary patch which seems to fix it, but I am not sure if I am doing the right thing. 0001-Only-advance-dataout-queue-if-there-is-space-in-it.patch