We were getting an assert "!secp256k1_fe_is_zero(&ge->x)", because
an all-zero pubkey is invalid. We allow marshal/unmarshal of NULL for
now, and clean up the error handling.
1. Use status_failed if master sends a bad message.
2. Similarly, kill the gossip daemon if it gives a bad reply.
3. Use an array for returned pubkeys: 0 or 2.
4. Use type_to_string(trc, struct short_channel_id, &scid) for tracing.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
cd90c8408c