avformat/dhav: Check avio_seek() return

Fixes: infinite loop
Fixes: 472567148/clusterfuzz-testcase-minimized-ffmpeg_dem_DHAV_fuzzer-4520943574908928

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

libavformat/dhav.c

@@ -321,7 +321,9 @@ static int dhav_read_header(AVFormatContext *s)
                 if (seek_back < 9)
                     break;
                 dhav->last_good_pos = avio_tell(s->pb);
-                avio_seek(s->pb, -seek_back, SEEK_CUR);
+                int64_t ret64 = avio_seek(s->pb, -seek_back, SEEK_CUR);
+                if (ret64 < 0)
+                    return ret64;
             }
             avio_seek(s->pb, dhav->last_good_pos, SEEK_SET);
         }