Error check number of characteristics

[Timmmm]

Currently it just blindly writes beyond the end of the array, leading to impossible-to-find bugs.

It doesn't help that the limit on the number of characteristics doesn't seem to be documented anywhere.

Sorry github's editor won't let me use spaces so there may be some correct indentation characters in there (i.e. tabs!). I did try.

[miklis]

Automatic CI verification build not done, please verify manually.

[rgrover]

thanks for contributing. this is good sanity checking. I'll review this more carefully shortly. Looks good based on an initial glance.

[Timmmm]

Thanks!