VC1 VDPAU: Mark missing reference frames as such.

Currently the code passes some nonsense values as
references instead, causing corruption with NVidia's
and assertion failures with Mesa's implementation.
For non-corrupted input this mostly happens in
interlaced bitstreams, e.g.
http://samples.mplayerhq.hu/V-codecs/WMV9/interlaced/480i30__codec_WVC1__mode_2__framerate_29.970__type_2__preproc_17.wmv.
The != VDP_INVALID handle assert does not trigger
(and probably is quite nonsense) because the frames
are initialized to 0.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
