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>

libavcodec/vdpau_vc1.c

@@ -44,14 +44,18 @@ static int vdpau_vc1_start_frame(AVCodecContext *avctx,
 
     switch (s->pict_type) {
     case AV_PICTURE_TYPE_B:
+        if (s->next_picture_ptr) {
         ref = ff_vdpau_get_surface_id(&s->next_picture);
         assert(ref != VDP_INVALID_HANDLE);
         info->backward_reference = ref;
+        }
         /* fall-through */
     case AV_PICTURE_TYPE_P:
+        if (s->last_picture_ptr) {
         ref = ff_vdpau_get_surface_id(&s->last_picture);
         assert(ref != VDP_INVALID_HANDLE);
         info->forward_reference  = ref;
+        }
     }
 
     info->slice_count       = 0;