vf_vdpaupp crashed on certain files (with --hwdec=vdpau --deinterlace).
This happened for example with mpeg2 files, which for some reason
typically contain some AVFrame side data. It turns out the last change
in 55c88fdb8f was not quite clean, and forgot the special cases in
mp_image_new_dummy_ref(). This function is supposed to copy all metadata
from the argument passed, except buffer refs. But there were new buffer
refs, that were not cleared properly. Also, the ff_side_data pointer
must be cleared, or the new mp_image would try to free it on
destruction.
The bottom line is that mp_image_new_dummy_ref() is a pretty bad idea,
and I suppose all callers with non-NULL arguments should be changed to
create a blank mp_image, and copy frame properties as needed (this
includes callers of mp_image_new_custom_ref()).
Fixes#5630.
e1b4e5e727