avformat/asfdec_o: Check offset before adding index entry

Fixes: signed integer overflow: 9223372036854550860 + 530259564 cannot be represented in type 'long'
Fixes: 49093/clusterfuzz-testcase-minimized-ffmpeg_dem_ASF_O_fuzzer-4697179192688640

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

@@ -888,6 +888,8 @@ static int asf_read_simple_index(AVFormatContext *s, const GUIDParseTable *g)
             av_log(s, AV_LOG_ERROR, "Skipping failed in asf_read_simple_index.\n");
             return offset;
         }
+        if (asf->first_packet_offset > INT64_MAX - asf->packet_size * pkt_num)
+            return AVERROR_INVALIDDATA;
         if (prev_pkt_num != pkt_num) {
             av_add_index_entry(st, asf->first_packet_offset + asf->packet_size *
                                pkt_num, av_rescale(interval, i, 10000),