FilesApp: guard getLocationInfo call

I saw some flakiness today in the QuickView UI test where a volume can
be unmounted whilst a rescan is in progress.  Then when it is finally
complete, this call to getLocationInfo fails.

Change-Id: Ie29e551b980b3e4bacea7b6bd39746c8c140e1aa
Reviewed-on: https://chromium-review.googlesource.com/c/1337136Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608277}

ui/file_manager/file_manager/foreground/js/directory_model.js

@@ -699,7 +699,7 @@ DirectoryModel.prototype.scan_ = function(
       var locationInfo =
           this.volumeManager_.getLocationInfo(
               assert(dirContents.getDirectoryEntry()));
-      var volumeInfo = locationInfo.volumeInfo;
+      var volumeInfo = locationInfo && locationInfo.volumeInfo;
       if (volumeInfo &&
           volumeInfo.volumeType === VolumeManagerCommon.VolumeType.DOWNLOADS &&
           locationInfo.isRootEntry) {