Fifo stability issue debugging

We observe some issues with missing trailers when the trailer is either empty (no hits, chipID 0 and CRC evaluates to 0), or if the CRC is just a single bit that can flip to 0 - which seems to happen sometimes. The work in this request fixes these issues at least for reasonable numbers of sent L1As, but more testing is needed for our DAQ system that can run into full FIFOs.