avcodec/ffwavesynth: Fix integer overflow with pink_ts_cur/next

Fixes: signed integer overflow: 6175076100092079360 - -5034989061050195840 cannot be represented in type 'long'
Fixes: 18614/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FFWAVESYNTH_fuzzer-5704508847423488

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

libavcodec/ffwavesynth.c

@@ -217,8 +217,8 @@ static void wavesynth_seek(struct wavesynth_context *ws, int64_t ts)
     *last = -1;
     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);
+        uint64_t pink_ts_cur  = (ws->cur_ts + PINK_UNIT - 1) & ~(PINK_UNIT - 1);
+        uint64_t pink_ts_next = ts & ~(PINK_UNIT - 1);
         int pos = ts & (PINK_UNIT - 1);
         lcg_seek(&ws->pink_state, (uint32_t)(pink_ts_next - pink_ts_cur) * 2);
         if (pos) {