jpeg: Consolidate error handling

Make all error paths in gdk_pixbuf__jpeg_image_load
go through the same path, so we don't forget to free
the exif data in half of them.
jpeg: apply icc profile when loading incrementally

This was apparently never tested, and thus didn't work.
Add a test for icc profile loading

This test verifies that the preceding commit indeed
makes icc-profile loading from jpeg files work.