The spec mandates this field be parsed using unsigned arithmetic. Nevertheless,
av1parser will use -1 apparently as an uninitialized value in
gst_av1_parse_frame_header. This immediately underflows last_frame_idx
though, since its type was defined as guint8. Fix this by converting to gint8.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2401>