avformat/iamf_parse: clear padding

Fixes: use of uninitialized value
Fixes: 70929/clusterfuzz-testcase-minimized-ffmpeg_dem_IAMF_fuzzer-5931276639469568

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

libavformat/iamf_parse.c

@@ -1081,6 +1081,7 @@ int ff_iamfdec_read_descriptors(IAMFContext *c, AVIOContext *pb,
         size = avio_read(pb, header, FFMIN(MAX_IAMF_OBU_HEADER_SIZE, max_size));
         if (size < 0)
             return size;
+        memset(header + size, 0, AV_INPUT_BUFFER_PADDING_SIZE);
 
         len = ff_iamf_parse_obu_header(header, size, &obu_size, &start_pos, &type, NULL, NULL);
         if (len < 0 || obu_size > max_size) {