Crash in NevisTPCUtilities decode_data method when corrupt data come up

[tomjunk]

In Run 18101, corrupt data started coming up in the Nevis TPC fragments. invalid fragment handles in the artROOT files from the DAQ were plentiful, but sometimes data got as far as the decoder method in NevisTPCUtilities.cc. If a word comes up with type kADC or kADCHuffman before any kChannelHeader word is found, then the uninitialized currentWaveformPtr gets used to store waveform data for which no channel has been identified, causing a segfault.

[tomjunk]

More details on the crash.

gdb --args lar -c run_decoders_job.fcl root://fndca1.fnal.gov:1094//pnfs/fnal.gov/usr/sbnd/archive/sbn/sbn_nd/data/raw/unknown/v1_10_04/sbnd_daq_v1_10_04/daq/00/01/81/01/data_EventBuilder1_art1_run18101_1_strmUnknown_20250107T214345.root

Program received signal SIGSEGV, Segmentation fault.
sbndaq::NevisTPCDecoder::decode_data (this=this@entry=0x7ffffffdd0b0, data_ptr=, n_words=n_words@entry=40593, wvfm_map=...)
at /home/trj/saut/triggerdecoder/sbndc4/srcs/sbndaq_artdaq_core/sbndaq-artdaq-core/Overlays/SBND/NevisTPC/NevisTPCUtilities.cc:97
97 currentWaveformPtr->emplace_back( currentWaveformPtr->back() + differences[i_diff - 1] );