avcodec/ffwavesynth: More correct cast in wavesynth_seek()

Fixes: signed integer overflow: 553590816 - -9223372036315799520 cannot be represented in type 'long'
Fixes: 15743/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FFWAVESYNTH_fuzzer-5705835377852416

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit f4605770af)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ffwavesynth.c

@@ -215,7 +215,7 @@ static void wavesynth_seek(struct wavesynth_context *ws, int64_t ts)
     ws->next_inter = i;
     ws->next_ts = i < ws->nb_inter ? ws->inter[i].ts_start : INF_TS;
     *last = -1;
-    lcg_seek(&ws->dither_state, (uint32_t)ts - ws->cur_ts);
+    lcg_seek(&ws->dither_state, (uint32_t)ts - (uint32_t)ws->cur_ts);
     if (ws->pink_need) {
         int64_t pink_ts_cur  = (ws->cur_ts + PINK_UNIT - 1) & ~(PINK_UNIT - 1);
         int64_t pink_ts_next = ts & ~(PINK_UNIT - 1);