avformat/matroskaenc: remove accidental variable shadowing

Should fix use of uninitialized value in a failure path.
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/matroskaenc.c

@@ -2697,8 +2697,8 @@ static int mkv_write_block(void *logctx, MatroskaMuxContext *mkv,
     if (side_data && side_data_size) {
         uint8_t *payload;
         size_t payload_size, buf_size;
-        int ret = av_dynamic_hdr_plus_to_t35((AVDynamicHDRPlus *)side_data, NULL,
-                                             &payload_size);
+        ret = av_dynamic_hdr_plus_to_t35((AVDynamicHDRPlus *)side_data, NULL,
+                                         &payload_size);
         if (ret < 0)
             return ret;