avformat/asfdec_o: Limit packet offset

avoids overflows with it

Fixes: signed integer overflow: 9223372036846866010 + 4294967047 cannot be represented in type 'long'
Fixes: 50993/clusterfuzz-testcase-minimized-ffmpeg_dem_ASF_O_fuzzer-6538296768987136
Fixes: 50993/clusterfuzz-testcase-minimized-ffmpeg_dem_ASF_O_fuzzer-657169555665715

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

libavformat/asfdec_o.c

@@ -1242,6 +1242,8 @@ static int asf_read_packet_header(AVFormatContext *s)
     unsigned char error_flags, len_flags, pay_flags;
 
     asf->packet_offset = avio_tell(pb);
+    if (asf->packet_offset > INT64_MAX/2)
+        asf->packet_offset = 0;
     error_flags = avio_r8(pb); // read Error Correction Flags
     if (error_flags & ASF_PACKET_FLAG_ERROR_CORRECTION_PRESENT) {
         if (!(error_flags & ASF_ERROR_CORRECTION_LENGTH_TYPE)) {