avcodec/ffwavesynth: Fix undefined overflow in wavesynth_synth_sample()

Fixes: signed integer overflow: 2147464192 + 21176 cannot be represented in type 'int'
Fixes: 19042/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FFWAVESYNTH_fuzzer-5719828090585088

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit fa47f6412d)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ffwavesynth.c

@@ -350,7 +350,8 @@ fail:
 static void wavesynth_synth_sample(struct wavesynth_context *ws, int64_t ts,
                                    int32_t *channels)
 {
-    int32_t amp, val, *cv;
+    int32_t amp, *cv;
+    unsigned val;
     struct ws_interval *in;
     int i, *last, pink;
     uint32_t c, all_ch = 0;