Don't allow continue from the connecting vertex of a closed way

Now that addNode tries to preserve circularity, this
could cause the continued segment to loop back.

And anyway it's confusing why one vertex allows it and none others do.

modules/operations/continue.js

@@ -15,6 +15,7 @@ export function operationContinue(selectedIDs, context) {
     function candidateWays() {
         return graph.parentWays(vertex).filter(function(parent) {
             return parent.geometry(graph) === 'line' &&
+                !parent.isClosed() &&
                 parent.affix(vertex.id) &&
                 (geometries.line.length === 0 || geometries.line[0] === parent);
         });