jpeg: Prevent crashes when stopping loading files with errors
jpeg: Prevent crashes when stopping loading files with errors

This time, we want to make sure to avoid accessing JPEG internals when
an error has already been set, in which case the file is irrecoverable.

Reproducer in pixbuf-randomly-modified, with file valid.2.jpeg and
seed R02S3d1f92e3076dbe16d2840cc408188f81

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf