Setting secondary structure of invalid segment crashes client.

[LociOiling]

As demonstrated by the recipe SSCrasher 1.0, attempting to set the secondary structure of an invalid segment crashes the client. This was observed on buildid 20191029-32048a6a12-win_x86-devprev, on puzzle 1784.

SSCrasher uses xpcall, and the specified cleanup routine gets called and does it's thing, but the client still crashes.