avformat/mxfdec: Check body_offset

Fixes: signed integer overflow: 538976288 - -9223372036315799520 cannot be represented in type 'long'
Fixes: 68060/clusterfuzz-testcase-minimized-ffmpeg_dem_MXF_fuzzer-5523457266745344

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

libavformat/mxfdec.c

@@ -791,6 +791,9 @@ static int mxf_read_partition_pack(void *arg, AVIOContext *pb, int tag, int size
     partition->index_sid = avio_rb32(pb);
     partition->body_offset = avio_rb64(pb);
     partition->body_sid = avio_rb32(pb);
+    if (partition->body_offset < 0)
+        return AVERROR_INVALIDDATA;
+
     if (avio_read(pb, op, sizeof(UID)) != sizeof(UID)) {
         av_log(mxf->fc, AV_LOG_ERROR, "Failed reading UID\n");
         return AVERROR_INVALIDDATA;