[gnome-epub-thumbnailer] (6 commits) ...epub: Work-around a broken ePub not getting a cover

[Bastien Nocera <hadess src gnome org>]

Summary of changes:

  8eadc8a... epub: Bail early when the cover path element doesn't exist (*)
  1be6c96... epub: Use g_autofree for root_path (*)
  56630b9... epub: Use g_autofree for root_file (*)
  dc5b88b... epub: Use g_autofree for metafile (*)
  18fa3a6... epub: Use g_autofree for cover_path (*)
  efd4343... epub: Work-around a broken ePub not getting a cover (*)

(*) This commit already existed in another branch; no separate mail sent