Do not overwrite VDPAU structures in ff_MPV_frame_start().

Fixes crashes with VDR and MPlayer as reported by irc user crow.
(cherry picked from commit 941b2240)

libavcodec/mpegvideo.c

@@ -1774,7 +1774,7 @@ int ff_MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
             return -1;
         }
 
-        if (!avctx->hwaccel) {
+        if (!avctx->hwaccel && !(avctx->codec->capabilities&CODEC_CAP_HWACCEL_VDPAU)) {
             for(i=0; i<avctx->height; i++)
                 memset(s->last_picture_ptr->f.data[0] + s->last_picture_ptr->f.linesize[0]*i,
                        0x80, avctx->width);