avcodec/ffwavesynth: Check sample rate before use

Fixes: division by zero
Fixes: 15725/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FFWAVESYNTH_fuzzer-5641231956180992

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 c95857a423)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ffwavesynth.c

@@ -270,7 +270,7 @@ static int wavesynth_parse_extradata(AVCodecContext *avc)
         dt = in->ts_end - in->ts_start;
         switch (in->type) {
             case WS_SINE:
-                if (edata_end - edata < 20)
+                if (edata_end - edata < 20 || avc->sample_rate <= 0)
                     return AVERROR(EINVAL);
                 f1  = AV_RL32(edata +  0);
                 f2  = AV_RL32(edata +  4);