avcodec/mpegvideo_enc: Use ptrdiff_t for stride

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit e063c1d079)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpegvideo_enc.c

@@ -1194,8 +1194,8 @@ static int load_input_picture(MpegEncContext *s, const AVFrame *pic_arg)
             }
 
             for (int i = 0; i < 3; i++) {
-                int src_stride = pic_arg->linesize[i];
-                int dst_stride = i ? s->uvlinesize : s->linesize;
+                ptrdiff_t src_stride = pic_arg->linesize[i];
+                ptrdiff_t dst_stride = i ? s->uvlinesize : s->linesize;
                 int h_shift = i ? s->chroma_x_shift : 0;
                 int v_shift = i ? s->chroma_y_shift : 0;
                 int w = s->width  >> h_shift;