avfilter/vf_dedot: Fix leak of AVFrame if making it writable fails

Even in this scenario, the frame still contains references to data that
won't be freed if the frame isn't unreferenced. And the AVFrame itself
will leak, too.

Fixes Coverity issue #1441422.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 212077eda46c4c3eb644774d2b1ccbeb3e322fff)

libavfilter/vf_dedot.c

@@ -312,7 +312,8 @@ static int activate(AVFilterContext *ctx)
                                                FFMIN(s->planeheight[2],
                                                ff_filter_get_nb_threads(ctx)));
                     }
-                }
+                } else
+                    av_frame_free(&out);
             } else if (!out) {
                 ret = AVERROR(ENOMEM);
             }