avcodec/utils: Check close before calling it

Fixes: NULL pointer dereference
Fixes: 15733/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_IDF_fuzzer-5658616977162240

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
static void ff_unlock_avcodec(const AVCodec *codec)
{
    if (!(codec->caps_internal & FF_CODEC_CAP_INIT_THREADSAFE) && codec->init)
        ff_mutex_unlock(&codec_mutex);
}

static void ff_unlock_avcodec(const AVCodec *codec)
{
    if (!(codec->caps_internal & FF_CODEC_CAP_INIT_THREADSAFE) && codec->init)
        ff_mutex_unlock(&codec_mutex);
}
