[Privet Filesystem] Fix null dereference

Fix null dereference issue in privet filesystem if listing a directory errors
out. Instead, signal an error.

BUG=

Review URL: https://codereview.chromium.org/180733006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255430 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255430