avcodec/mpegvideo: fix buffer clear code so it should work with negative linesizes

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

libavcodec/mpegvideo.c

@@ -1774,14 +1774,15 @@ int ff_MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
             return -1;
         }
 
-        memset(s->last_picture_ptr->f.data[0], 0x80,
-               avctx->height * s->last_picture_ptr->f.linesize[0]);
-        memset(s->last_picture_ptr->f.data[1], 0x80,
-               (avctx->height >> v_chroma_shift) *
-               s->last_picture_ptr->f.linesize[1]);
-        memset(s->last_picture_ptr->f.data[2], 0x80,
-               (avctx->height >> v_chroma_shift) *
-               s->last_picture_ptr->f.linesize[2]);
+        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);
+        for(i=0; i<FF_CEIL_RSHIFT(avctx->height, v_chroma_shift); i++) {
+            memset(s->last_picture_ptr->f.data[1] + s->last_picture_ptr->f.linesize[1]*i,
+                   0x80, FF_CEIL_RSHIFT(avctx->width, h_chroma_shift));
+            memset(s->last_picture_ptr->f.data[2] + s->last_picture_ptr->f.linesize[2]*i,
+                   0x80, FF_CEIL_RSHIFT(avctx->width, h_chroma_shift));
+        }
 
         if(s->codec_id == AV_CODEC_ID_FLV1 || s->codec_id == AV_CODEC_ID_H263){
             for(i=0; i<avctx->height; i++)