Memset svc_params in vp9 wrapper to avoid undefined behavior.

Due to a recent interface change for svc_params in vp9 svc, which
allows speed setting per layer, svc_params should be inited to 0
for safety.

Review-Url: https://codereview.webrtc.org/2179753003
Cr-Commit-Position: refs/heads/master@{#13561}

webrtc/modules/video_coding/codecs/vp9/vp9_impl.cc

@@ -81,6 +81,7 @@ VP9EncoderImpl::VP9EncoderImpl()
       // Use two spatial when screensharing with flexible mode.
       spatial_layer_(new ScreenshareLayersVP9(2)) {
   memset(&codec_, 0, sizeof(codec_));
+  memset(&svc_internal_.svc_params, 0, sizeof(vpx_svc_extra_cfg_t));
   uint32_t seed = rtc::Time32();
   srand(seed);
 }