cbs_h264: Fix memory leak in error case

Fixes CID 1419834.

libavcodec/cbs_h2645.c

@@ -824,6 +824,7 @@ static int cbs_h264_read_nal_unit(CodedBitstreamContext *ctx,
             err = cbs_h264_read_sei(ctx, &gbc, sei);
             if (err < 0) {
                 cbs_h264_free_sei(sei);
+                av_free(sei);
                 return err;
             }