Fix up gdk_pixbuf_new_from_stream_async()
Fix up gdk_pixbuf_new_from_stream_async()

Add fixes for the various issues brought up in
https://bugzilla.gnome.org/show_bug.cgi?id=790584

 * g_task_propagate_pointer() is transfer full
 * GBytes get leaked
 * always pass the stream as task source object to avoid criticals
 * don't check the source object in finish()

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