avformat/dhav: Use 64bit seek_back

Fixes: negation of -2147483648 cannot be represented in type 'int'; cast to an unsigned type to negate this value to itself
Fixes: 50993/clusterfuzz-testcase-minimized-ffmpeg_dem_DHAV_fuzzer-6604736532447232

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

libavformat/dhav.c

@@ -242,7 +242,7 @@ static int64_t get_duration(AVFormatContext *s)
     avio_seek(s->pb, avio_size(s->pb) - 8, SEEK_SET);
     while (avio_tell(s->pb) > 12 && max_interations--) {
         if (avio_rl32(s->pb) == MKTAG('d','h','a','v')) {
-            int seek_back = avio_rl32(s->pb);
+            int64_t seek_back = avio_rl32(s->pb);
 
             avio_seek(s->pb, -seek_back, SEEK_CUR);
             read_chunk(s);