avcodec/ffwavesynth: Avoid signed integer overflow in phi_at()

Fixes: signed integer overflow: 2314885530818453536 - -9070214327174160352 cannot be represented in type 'long'
Fixes: 31000/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FFWAVESYNTH_fuzzer-6558389742206976

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>

libavcodec/ffwavesynth.c

@@ -188,7 +188,7 @@ static uint64_t frac64(uint64_t a, uint64_t b)
 
 static uint64_t phi_at(struct ws_interval *in, int64_t ts)
 {
-    uint64_t dt = ts - in->ts_start;
+    uint64_t dt = ts - (uint64_t)in->ts_start;
     uint64_t dt2 = dt & 1 ? /* dt * (dt - 1) / 2 without overflow */
                    dt * ((dt - 1) >> 1) : (dt >> 1) * (dt - 1);
     return in->phi0 + dt * in->dphi0 + dt2 * in->ddphi;