avcodec/mpegvideo: Fix null pointer dereference with grayscale decoding with...

avcodec/mpegvideo: Fix null pointer dereference with grayscale decoding with mpeg2_field_encoding.ts
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpegvideo.c

@@ -1896,11 +1896,13 @@ 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,
                        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->last_picture_ptr->f->data[2]) {
+                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){