avcodec/ffwavesynth: Correct undefined overflow of PINK_UNIT

Fixes: signed integer overflow: 9223372036854775775 + 128 cannot be represented in type 'long'
Fixes: 20054/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FFWAVESYNTH_fuzzer-5686385113825280

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

@@ -217,7 +217,7 @@ 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) {
-        uint64_t pink_ts_cur  = (ws->cur_ts + PINK_UNIT - 1) & ~(PINK_UNIT - 1);
+        uint64_t pink_ts_cur  = (ws->cur_ts + (uint64_t)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);