Fix 'cleanupIDs' minify bug due to preserved IDs

Prevent 'cleanupIDs' plugin from producing a preserved ID, including one
which matches a preserved prefix, when minifying.

Closes #1158

plugins/cleanupIDs.js

@@ -121,13 +121,18 @@ exports.fn = function(data, params) {
         return data;
     }
 
+    const idPreserved = id => preserveIDs.has(id) || idMatchesPrefix(preserveIDPrefixes, id);
+
     for (var ref of referencesIDs) {
         var key = ref[0];
 
         if (IDs.has(key)) {
             // replace referenced IDs with the minified ones
-            if (params.minify && !preserveIDs.has(key) && !idMatchesPrefix(preserveIDPrefixes, key)) {
-                currentIDstring = getIDstring(currentID = generateID(currentID), params);
+            if (params.minify && !idPreserved(key)) {
+                do {
+                    currentIDstring = getIDstring(currentID = generateID(currentID), params);
+                } while (idPreserved(currentIDstring));
+
                 IDs.get(key).attr('id').value = currentIDstring;
 
                 for (var attr of ref[1]) {
@@ -143,7 +148,7 @@ exports.fn = function(data, params) {
     // remove non-referenced IDs attributes from elements
     if (params.remove) {
         for(var keyElem of IDs) {
-            if (!preserveIDs.has(keyElem[0]) && !idMatchesPrefix(preserveIDPrefixes, keyElem[0])) {
+            if (!idPreserved(keyElem[0])) {
                 keyElem[1].removeAttr('id');
             }
         }