avcodec/get_buffer: Use av_buffer_mallocz() for audio same as its done for video

Fixes: Use of uninintialized value
Fixes: 70993/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AAC_LATM_fuzzer-6378949754552320
Fixes: 71104/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AAC_FIXED_fuzzer-5001538727116800

For the AAC/USAC/SBR code which reads uninitialized memory, it would be good, if it did not
a fix for that is welcome!

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/get_buffer.c

@@ -142,7 +142,10 @@ static int update_frame_pool(AVCodecContext *avctx, AVFrame *frame)
         if (ret < 0)
             goto fail;
 
-        pool->pools[0] = av_buffer_pool_init(pool->linesize[0], NULL);
+        pool->pools[0] = av_buffer_pool_init(pool->linesize[0],
+                                             CONFIG_MEMORY_POISONING ?
+                                                NULL :
+                                                av_buffer_allocz);
         if (!pool->pools[0]) {
             ret = AVERROR(ENOMEM);
             goto fail;