avformat/isom: don't free extradata before calling ff_get_extradata()

ff_get_extradata() frees any existing extradata before allocating now,
and using av_free() here leaves a dangling pointer that will result in
a double free.

Fixes a regression since 0ca33b1d.
Tested-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/isom.c

@@ -526,7 +526,6 @@ FF_ENABLE_DEPRECATION_WARNINGS
         av_log(fc, AV_LOG_TRACE, "Specific MPEG-4 header len=%d\n", len);
         if (!len || (uint64_t)len > (1<<30))
             return AVERROR_INVALIDDATA;
-        av_free(st->codecpar->extradata);
         if ((ret = ff_get_extradata(fc, st->codecpar, pb, len)) < 0)
             return ret;
         if (st->codecpar->codec_id == AV_CODEC_ID_AAC) {