mpegvideo: fix overwriting hwaccel surface objects

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpegvideo.c

@@ -1774,6 +1774,7 @@ int ff_MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
             return -1;
         }
 
+        if (!avctx->hwaccel) {
         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);
@@ -1788,6 +1789,7 @@ int ff_MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
             for(i=0; i<avctx->height; i++)
             memset(s->last_picture_ptr->f.data[0] + s->last_picture_ptr->f.linesize[0]*i, 16, avctx->width);
         }
+        }
 
         ff_thread_report_progress(&s->last_picture_ptr->tf, INT_MAX, 0);
         ff_thread_report_progress(&s->last_picture_ptr->tf, INT_MAX, 1);