Change so that the handling of NAL unit that can't be parsed when using
AU alignment is the same as when using NAL alignment, ie drop the data
if it can't be parsed.
If the AU contains more than one NAL unit any correctly parsed NAL unit
in the AU is kept.
Fixes#4436
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8976>