avformat/avidec: fix position overflow in avi_load_index()

Fixes: signed integer overflow: 9223372033098784808 + 4294967072 cannot be represented in type 'long'
Fixes: 29102/clusterfuzz-testcase-minimized-ffmpeg_dem_AVI_fuzzer-6732488912273408

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

libavformat/avidec.c

@@ -1776,7 +1776,10 @@ static int avi_load_index(AVFormatContext *s)
         size = avio_rl32(pb);
         if (avio_feof(pb))
             break;
-        next = avio_tell(pb) + size + (size & 1);
+        next = avio_tell(pb);
+        if (next < 0 || next > INT64_MAX - size - (size & 1))
+            break;
+        next += size + (size & 1LL);
 
         if (tag == MKTAG('i', 'd', 'x', '1') &&
             avi_read_idx1(s, size) >= 0) {