on with almost no work, only
  sometimes falling back to the full, expensive reset that is always required
  when the stream is in an indeterminate state (i.e., the reset performed by
  LZ4_resetStream()).

  LZ4_streams are guaranteed to be in a valid state when:
  - returned from LZ4_createStream()
  - reset by LZ4_resetStream()
  - memset(stream, 0, sizeof(LZ4_stream_t)), though this is discouraged
  - the stream was in a valid state and was reset by LZ4_resetStream_fast()
  - the stream was in a valid s