channeld/lightningd/hsmd: strengthen our checks against 0-output txs.

If we ever do this, we'd end up with an unspendable commitment tx anyway.
It might be able to happen if we have htlcs added from the non-fee-paying
party while the fees are increased, though.  But better to close the
channel and get a report about it if that happens.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

channeld/commit_tx.c

@@ -242,6 +242,14 @@ struct bitcoin_tx *commit_tx(const tal_t *ctx,
 		n++;
 	}
 
+	/* BOLT #2:
+	 *
+	 *  - MUST set `channel_reserve_satoshis` greater than or equal to
+	 *    `dust_limit_satoshis`.
+	 */
+	/* This means there must be at least one output. */
+	assert(n > 0);
+
 	assert(n <= tx->wtx->outputs_allocation_len);
 	tal_resize(htlcmap, n);