[SyncFS] Fix NULL pointer access on DriveMetadataStore error case

TEST=manual
NOTRY=true

Review URL: https://codereview.chromium.org/26539007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228082 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228082