Fix failed assertion tearing down offline tests

Discovered while working on #894

Change-Id: Icf74cd4bf804df0b055033f9c1113971e1644202

lib/offline/db_engine.js

@@ -246,7 +246,7 @@ shaka.offline.DBEngine.prototype.createTransaction_ = function(storeName,
                                                                type,
                                                                action) {
 
-  goog.asserts.assert(this.db_, 'Must not be destroyed');
+  goog.asserts.assert(this.db_, 'DBEngine must not be destroyed');
   goog.asserts.assert(type == 'readonly' || type == 'readwrite',
                       'Type must be "readonly" or "readwrite"');
 

lib/offline/download_manager.js

@@ -127,7 +127,12 @@ shaka.offline.DownloadManager.prototype.destroy = function() {
   var storage = this.storageEngine_;
   var segments = this.storedSegments_;
   var p = this.promise_ || Promise.resolve();
-  p = p.then(function() { return storage.removeKeys('segment', segments); });
+
+  // Don't try to remove segments if there are none.  That may trigger an error
+  // in storage if the DB connection was never created.
+  if (segments.length) {
+    p = p.then(function() { return storage.removeKeys('segment', segments); });
+  }
 
   // Don't destroy() storageEngine since it is owned by Storage.
 
@@ -229,7 +234,8 @@ shaka.offline.DownloadManager.prototype.downloadAndStore = function(manifest) {
  * @private
  */
 shaka.offline.DownloadManager.prototype.downloadSegment_ = function(segment) {
-  goog.asserts.assert(this.retryParams_, 'Must not be destroyed');
+  goog.asserts.assert(this.retryParams_,
+                      'DownloadManager must not be destroyed');
   var type = shaka.net.NetworkingEngine.RequestType.SEGMENT;
   var request =
       shaka.net.NetworkingEngine.makeRequest(segment.uris, this.retryParams_);

lib/offline/storage.js

@@ -153,7 +153,7 @@ shaka.offline.Storage.prototype.destroy = function() {
  * @export
  */
 shaka.offline.Storage.prototype.configure = function(config) {
-  goog.asserts.assert(this.config_, 'Must not be destroyed');
+  goog.asserts.assert(this.config_, 'Storage must not be destroyed');
   shaka.util.ConfigUtils.mergeConfigObjects(
       this.config_, config, this.defaultConfig_(), {}, '');
 };