failure to process certain DNG files (Leica M246)

Darktable/Rawspeed handle about 95% of the DNG files from a Leica M Monochrom (Typ 246) just fine, but fail on some of them. The failing files can be opened in RawTherapee, so this doesn't seem to be a matter of data corruption.

Using the rstest utility produces some output:

L1000625.DNG failed: /home/jeremy/dev/git/rawspeed/src/librawspeed/decompressors/AbstractDngDecompressor.cpp:224: void rawspeed::AbstractDngDecompressor::decompress() const: Too many errors encountered. Giving up. First Error:
/home/jeremy/dev/git/rawspeed/src/librawspeed/io/Buffer.h:173: const uint8_t* rawspeed::Buffer::getData(rawspeed::Buffer::size_type, rawspeed::Buffer::size_type) const: Buffer overflow: image file may be truncated
Total decoding time: 0s

If there are other diagnostics I can run, I'd be happy to do so. I can of course provide an offending DNG file as well (they tend to be on the order of 20 MB).

Start by obtaining one of the offending Leica M246 DNG files and reproducing the failure with the rstest utility. Trace the reported error through src/librawspeed/decompressors/AbstractDngDecompressor.cpp and src/librawspeed/io/Buffer.h, then identify the cause of the buffer overflow and add or update coverage so the affected file decodes successfully.