This bug is not fixed in version 0.5.1-3 and can still be reproduced using the exact same steps as described above (and below). In fact I had already verified that it still occurs in upstream 0.5.0 (see message #10).
$ sox -n sine.wav synth 1 sine 1000
$ ffmpeg -i sine.wav sine.mp3
$ bs1770gain sine.mp3
analyzing ...
[1/1] "sine.mp3": Segmentation fault (core dumped)
