ffmpeg: use av_freep() for forced_kf_pts

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -3259,7 +3259,7 @@ static int transcode(void)
                     ost->logfile = NULL;
                 }
                 av_freep(&ost->st->codec->subtitle_header);
-                av_free(ost->forced_kf_pts);
+                av_freep(&ost->forced_kf_pts);
                 av_dict_free(&ost->opts);
                 av_dict_free(&ost->swr_opts);
                 av_dict_free(&ost->resample_opts);