ld: Add function to retrieve either the scid or the local alias

We use this in a couple of places, when we want to refer to a channel
by its `short_channel_id`, I'm moving this into a separate function
primarily to have a way to mark places where we do that.

lightningd/channel.c

@@ -956,3 +956,13 @@ void channel_fail_reconnect(struct channel *channel, const char *fmt, ...)
 	err_and_reconnect(channel, tal_vfmt(tmpctx, fmt, ap), 1);
 	va_end(ap);
 }
+
+const struct short_channel_id *
+channel_scid_or_local_alias(const struct channel *chan)
+{
+	assert(chan->scid != NULL || chan->alias[LOCAL] != NULL);
+	if (chan->scid != NULL)
+		return chan->scid;
+	else
+		return chan->alias[LOCAL];
+}

lightningd/channel.h

@@ -476,6 +476,16 @@ static inline bool channel_has(const struct channel *channel, int f)
 	return channel_type_has(channel->type, f);
 }
 
+/**
+ * Either returns the short_channel_id if it is known or the local alias.
+ *
+ * This is used to refer to a channel by its scid. But sometimes we
+ * don't have a scid yet, e.g., for `zeroconf` channels, so we resort
+ * to referencing it by the local alias, which we have in that case.
+ */
+const struct short_channel_id *
+channel_scid_or_local_alias(const struct channel *chan);
+
 void get_channel_basepoints(struct lightningd *ld,
 			    const struct node_id *peer_id,
 			    const u64 dbid,