Fix potential infinite async loop

Related issue:
https://bugzilla.mozilla.org/show_bug.cgi?id=1929326

As identified by @Rob--W:
https://bugzilla.mozilla.org/show_bug.cgi?id=1929326#c9

Truncated or otherwise corrupted asset content in extension storage
could lead to infinite async loop causing high CPU usage in uBO and
its workers.

Likely related to the issue of the asset content returned as
`undefined`:
https://github.com/gorhill/uBlock/blob/652f1787878ba434c3a65287afcd84082c409397/src/js/cachestorage.js#L98

src/js/assets.js

@@ -19,18 +19,15 @@
     Home: https://github.com/gorhill/uBlock
 */
 
-'use strict';
-
-/******************************************************************************/
+import * as sfp from './static-filtering-parser.js';
 
-import µb from './background.js';
 import { broadcast } from './broadcast.js';
 import cacheStorage from './cachestorage.js';
-import { ubolog } from './console.js';
 import { i18n$ } from './i18n.js';
 import logger from './logger.js';
-import * as sfp from './static-filtering-parser.js';
-import { orphanizeString, } from './text-utils.js';
+import { orphanizeString } from './text-utils.js';
+import { ubolog } from './console.js';
+import µb from './background.js';
 
 /******************************************************************************/
 
@@ -50,6 +47,9 @@ let remoteServerFriendly = false;
 
 /******************************************************************************/
 
+const hasOwnProperty = (o, p) =>
+    Object.prototype.hasOwnProperty.call(o, p);
+
 const stringIsNotEmpty = s => typeof s === 'string' && s !== '';
 
 const parseExpires = s => {
@@ -107,8 +107,8 @@ const resourceTimeFromXhr = xhr => {
 
 const resourceTimeFromParts = (parts, time) => {
     const goodParts = parts.filter(part => typeof part === 'object');
-    return goodParts.reduce((acc, part) =>
-        ((part.resourceTime || 0) > acc ? part.resourceTime : acc),
+    return goodParts.reduce(
+        (acc, part) => ((part.resourceTime || 0) > acc ? part.resourceTime : acc),
         time
     );
 };
@@ -246,6 +246,7 @@ const fireNotification = function(topic, details) {
 assets.fetch = function(url, options = {}) {
     return new Promise((resolve, reject) => {
     // Start of executor
+    /* eslint-disable indent */
 
     const timeoutAfter = µb.hiddenSettings.assetFetchTimeout || 30;
     const xhr = new XMLHttpRequest();
@@ -322,6 +323,7 @@ assets.fetch = function(url, options = {}) {
         onErrorEvent.call(xhr);
     }
 
+    /* eslint-enable indent */
     // End of executor
     });
 };
@@ -733,7 +735,7 @@ async function assetCacheRead(assetKey, updateReadTime = false) {
     }
 
     if ( bin instanceof Object === false ) { return reportBack(''); }
-    if ( bin.hasOwnProperty(internalKey) === false ) { return reportBack(''); }
+    if ( hasOwnProperty(bin, internalKey) === false ) { return reportBack(''); }
 
     const entry = assetCacheRegistry[assetKey];
     if ( entry === undefined ) { return reportBack(''); }
@@ -1277,7 +1279,9 @@ async function diffUpdater() {
             if ( data.status === 'needtext' ) {
                 ubolog('Diff updater: need text for', data.assetKey);
                 assetCacheRead(data.assetKey).then(result => {
-                    data.text = result.content;
+                    // https://bugzilla.mozilla.org/show_bug.cgi?id=1929326#c9
+                    //   Must never be set to undefined!
+                    data.text = result.content || '';
                     data.status = undefined;
                     checkAndCorrectDiffPath(data);
                     bc.postMessage(data);