avformat/hls: Use unsigned for iv computation

Fixes: signed integer overflow: 9223372036854775748 + 60 cannot be represented in type 'long'
Fixes: 44417/clusterfuzz-testcase-minimized-ffmpeg_dem_HLS_fuzzer-5802443881971712

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

libavformat/hls.c

@@ -914,7 +914,7 @@ static int parse_playlist(HLSContext *c, const char *url,
                 if (has_iv) {
                     memcpy(seg->iv, iv, sizeof(iv));
                 } else {
-                    int64_t seq = pls->start_seq_no + pls->n_segments;
+                    uint64_t seq = pls->start_seq_no + (uint64_t)pls->n_segments;
                     memset(seg->iv, 0, sizeof(seg->iv));
                     AV_WB64(seg->iv + 8, seq);
                 }