lnwire: add *OpaqueAddrs case in WriteElements

And then expand the chanbackup unit tests to cover such a case.

chanbackup/backup_test.go

@@ -157,6 +157,7 @@ func TestFetchStaticChanBackups(t *testing.T) {
 	chanSource.addAddrsForNode(randomChan1.IdentityPub, []net.Addr{addr1})
 	chanSource.addAddrsForNode(randomChan2.IdentityPub, []net.Addr{addr2})
 	chanSource.addAddrsForNode(randomChan2.IdentityPub, []net.Addr{addr3})
+	chanSource.addAddrsForNode(randomChan2.IdentityPub, []net.Addr{addr4})
 
 	// With the channel source populated, we'll now attempt to create a set
 	// of backups for all the channels. This should succeed, as all items