channeldb: sent internal db pointer in FetchOpenChannels

This commit fixes a bug which would potentially cause a panic if a
channel returned from FetchOpenChannels attempted to access the
internal pointer to the database.

To fix this bug, the pointer is now properly set once the channel has
been loaded from the database.

channeldb/db.go

@@ -177,6 +177,7 @@ func (d *DB) FetchOpenChannels(nodeID *wire.ShaHash) ([]*OpenChannel, error) {
 			if err != nil {
 				return err
 			}
+			oChannel.Db = d
 
 			channels = append(channels, oChannel)
 			return nil