avfilter/vf_morpho: Fix leak of output frame on error

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/vf_morpho.c

@@ -932,6 +932,7 @@ copy:
     out->pts = av_rescale_q(s->fs.pts, s->fs.time_base, outlink->time_base);
     return ff_filter_frame(outlink, out);
 fail:
+    av_frame_free(&out);
     av_frame_free(&in);
     return ret;
 }