avformat/avs: Use 64bit for the avio_tell() output

Fixes: signed integer overflow: 9223372036854775807 - -1 cannot be represented in type 'long'
Fixes: 26549/clusterfuzz-testcase-minimized-ffmpeg_dem_AVS_fuzzer-4844306424397824

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

libavformat/avs.c

@@ -128,7 +128,8 @@ avs_read_video_packet(AVFormatContext * s, AVPacket * pkt,
 static int avs_read_audio_packet(AVFormatContext * s, AVPacket * pkt)
 {
     AvsFormat *avs = s->priv_data;
-    int ret, size;
+    int ret;
+    int64_t size;
 
     size = avio_tell(s->pb);
     ret = ff_voc_get_packet(s, pkt, avs->st_audio, avs->remaining_audio_size);