avcodec/jpeg2000dec: clear pointer which become stale in get_ppt()

Fixes: use after free
Fixes: 22484/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_JPEG2000_fuzzer-5671488765296640

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

libavcodec/jpeg2000dec.c

@@ -928,6 +928,7 @@ static int get_ppt(Jpeg2000DecoderContext *s, int n)
         tile->packed_headers = new;
     } else
         return AVERROR(ENOMEM);
+    memset(&tile->packed_headers_stream, 0, sizeof(tile->packed_headers_stream));
     memcpy(tile->packed_headers + tile->packed_headers_size,
            s->g.buffer, n - 3);
     tile->packed_headers_size += n - 3;