avformat/isom: zero MPEG4AudioConfig to avoid leaving fields uninitialized

Fixes: use of uninitialized memory
Fixes: msan_uninit-mem_7f392c443e0d_1433_Crowd_Applause_01.caf
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/isom.c

@@ -461,7 +461,7 @@ int ff_mp4_read_dec_config_descr(AVFormatContext *fc, AVStream *st, AVIOContext
             return AVERROR(ENOMEM);
         avio_read(pb, st->codec->extradata, len);
         if (st->codec->codec_id == AV_CODEC_ID_AAC) {
-            MPEG4AudioConfig cfg;
+            MPEG4AudioConfig cfg = {0};
             avpriv_mpeg4audio_get_config(&cfg, st->codec->extradata,
                                          st->codec->extradata_size * 8, 1);
             st->codec->channels = cfg.channels;