graph/db: fix edges bucket check

This commit fixes a bug that would check that the passed `edge` argument
of UpdateChannelEdge is nil but it should actually be checking if the
`edges` bucket is nil.

Author: ellemouton

graph/db/graph.go

@@ -1309,7 +1309,7 @@ func (c *ChannelGraph) UpdateChannelEdge(edge *models.ChannelEdgeInfo) error {
 
 	return kvdb.Update(c.db, func(tx kvdb.RwTx) error {
 		edges := tx.ReadWriteBucket(edgeBucket)
-		if edge == nil {
+		if edges == nil {
 			return ErrEdgeNotFound
 		}