Created attachment 5481 Patch fixing the issue. When there is no error, the pointer pointed by error should be assigned NULL, not error itself as it doesn't change anything outside the function. I attach a fix.
commit dc74b5d38009a5f6c1064f69d8b7d632ae0b52a5 Author: Jannis Pohlmann <jannis@xfce.org> Date: Fri Jul 25 13:58:32 2014 +0200 Fix setting internal error to NULL in pixbuf thumbnailer (bug #10878)
The patch is not correct, this idea of setting error to NULL to ignore further errors.
Reverted the commit and added comments.