avfilter/vf_vidstabdetect: Avoid double AVERRORS

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit bb04235d72)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/vf_vidstabdetect.c

@@ -166,7 +166,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
     }
     if (vsMotionDetection(md, &localmotions, &frame) != VS_OK) {
         av_log(ctx, AV_LOG_ERROR, "motion detection failed");
-        return AVERROR(AVERROR_EXTERNAL);
+        return AVERROR_EXTERNAL;
     } else {
         if (vsWriteToFile(md, s->f, &localmotions) != VS_OK) {
             int ret = AVERROR(errno);