Make sure we're rejecting 0-amount HTLC sends

[TheBlueMatt]

I don't think we are, but they're not allowed.

[carlaKC]

Rejected in send_htlc and update_add_htlc. AFAIK those are the only paths to add a HTLC so seems good?

[TheBlueMatt]

Oh, yea, I guess I was surprised it wasn't in the higher-level send pipeline, tho. I imagine doing it in the channel state machine will result in a weird error (like a timeout/retries-exhausted error or something). Probably worth adding higher up.