libavformat/iff: Check for overflow in body_end calculation

Fixes: signed integer overflow: -6322983228386819992 - 5557477266266529857 cannot be represented in type 'long'
Fixes: 50112/clusterfuzz-testcase-minimized-ffmpeg_dem_IFF_fuzzer-6329186221948928

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

libavformat/iff.c

@@ -501,6 +501,9 @@ static int iff_read_header(AVFormatContext *s)
         case ID_DST:
         case ID_MDAT:
             iff->body_pos = avio_tell(pb);
+            if (iff->body_pos < 0 || iff->body_pos + data_size > INT64_MAX)
+                return AVERROR_INVALIDDATA;
+
             iff->body_end = iff->body_pos + data_size;
             iff->body_size = data_size;
             if (chunk_id == ID_DST) {