avfilter/vf_scale_qsv: add logging context to log

Signed-off-by: Steven Liu <lq@chinaffmpeg.org>

libavfilter/vf_scale_qsv.c

@@ -541,7 +541,7 @@ static int qsvscale_config_props(AVFilterLink *outlink)
     return 0;
 
 fail:
-    av_log(NULL, AV_LOG_ERROR,
+    av_log(ctx, AV_LOG_ERROR,
            "Error when evaluating the expression '%s'\n", expr);
     return ret;
 }