`combinePubKeys []` crashes

[ysangkok]

The empty list must be handled, otherwise libsecp256k1 will abort(3).

[jprupp]

I'll take a pull request for that too, and do a release in a few days.

[ysangkok]

@jprupp Do you prefer it taking a Data.List.NonEmpty (which is in base), or return Nothing?

[jprupp]

Definitely return Nothing.