fixup! gossipd: use simple inline array for nodes with few channels.

Suggested-by: @cdecker
Suggested-by: @niftynei

gossipd/routing.c

@@ -79,7 +79,7 @@ static struct node_map *empty_node_map(const tal_t *ctx)
 	return map;
 }
 
-/* We use a simple array until we have too many. */
+/* We use a simple array (with NULL entries) until we have too many. */
 static bool node_uses_chan_map(const struct node *node)
 {
 	/* This is a layering violation: last entry in htable is the table ptr,

gossipd/routing.h

@@ -105,6 +105,9 @@ static inline bool chan_eq_scid(const struct chan *c,
 }
 HTABLE_DEFINE_TYPE(struct chan, chan_map_scid, hash_scid, chan_eq_scid, chan_map);
 
+/* For a small number of channels (by far the most common) we use a simple
+ * array, with empty buckets NULL.  For larger, we use a proper hash table,
+ * with the extra allocation that implies. */
 #define NUM_IMMEDIATE_CHANS (sizeof(struct chan_map) / sizeof(struct chan *) - 1)
 
 struct node {