[xml-fold addon] Improve handling of line-broken opening tags

Issue codemirror#5179

Author: marijnh

addon/fold/xml-fold.js

@@ -137,12 +137,14 @@
   CodeMirror.registerHelper("fold", "xml", function(cm, start) {
     var iter = new Iter(cm, start.line, 0);
     for (;;) {
-      var openTag = toNextTag(iter), end;
-      if (!openTag || !(end = toTagEnd(iter)) || iter.line != start.line) return;
+      var openTag = toNextTag(iter)
+      if (!openTag || iter.line != start.line) return
+      var end = toTagEnd(iter)
+      if (!end) return
       if (!openTag[1] && end != "selfClose") {
         var startPos = Pos(iter.line, iter.ch);
         var endPos = findMatchingClose(iter, openTag[2]);
-        return endPos && {from: startPos, to: endPos.from};
+        return endPos && cmp(endPos.from, startPos) > 0 ? {from: startPos, to: endPos.from} : null
       }
     }
   });