avformat/isom: Uninit layout in ff_mp4_read_dec_config_descr()

Fixes: memleak
Fixes: 67442/clusterfuzz-testcase-minimized-ffmpeg_dem_CAF_fuzzer-5068813261406208

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>
(cherry picked from commit d157725cf7)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/isom.c

@@ -358,6 +358,7 @@ int ff_mp4_read_dec_config_descr(AVFormatContext *fc, AVStream *st, AVIOContext
                                                 st->codecpar->extradata_size, 1, fc);
             if (ret < 0)
                 return ret;
+            av_channel_layout_uninit(&st->codecpar->ch_layout);
             st->codecpar->ch_layout.order = AV_CHANNEL_ORDER_UNSPEC;
             st->codecpar->ch_layout.nb_channels = cfg.channels;
             if (cfg.object_type == 29 && cfg.sampling_index < 3) // old mp3on4