The error is fixed by commit 54fd92bc (util/cbfstool/lz4frame.c: Add comment to fall through).
Looks like I'm too late for that commit, but in general, please do not just hack around in the LZ4 files. That code was pulled in verbatim from the upstream source -- if there are issues with it, please instead send a patch to https://github.com/lz4/lz4 and resync our code base to there once it has landed.