avformat/iamf_reader: Initialize padding and check read in ff_iamf_read_packet()

Fixes: Use of uninitialized memory
Fixes: 377642312/clusterfuzz-testcase-minimized-ffmpeg_dem_IAMF_fuzzer-4554550985424896

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit aec2933344b2b32fc931bdf0b46eef1bd42225ff)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/iamf_reader.c

@@ -282,7 +282,7 @@ int ff_iamf_read_packet(AVFormatContext *s, IAMFDemuxContext *c,
     int read = 0;
 
     while (1) {
-        uint8_t header[MAX_IAMF_OBU_HEADER_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];
+        uint8_t header[MAX_IAMF_OBU_HEADER_SIZE + AV_INPUT_BUFFER_PADDING_SIZE] = {0};
         enum IAMF_OBU_Type type;
         unsigned obu_size;
         unsigned skip_samples, discard_padding;
@@ -294,6 +294,8 @@ int ff_iamf_read_packet(AVFormatContext *s, IAMFDemuxContext *c,
         size = avio_read(pb, header, FFMIN(MAX_IAMF_OBU_HEADER_SIZE, max_size));
         if (size < 0)
             return size;
+        if (size != FFMIN(MAX_IAMF_OBU_HEADER_SIZE, max_size))
+            return AVERROR_INVALIDDATA;
 
         len = ff_iamf_parse_obu_header(header, size, &obu_size, &start_pos, &type,
                                        &skip_samples, &discard_padding);