Avoid undefined behavior in vp8 screenshare_layers

webrtc/modules/video_coding/codecs/vp8/screenshare_layers.cc

Index: webrtc/modules/video_coding/codecs/vp8/screenshare_layers.cc
diff --git a/webrtc/modules/video_coding/codecs/vp8/screenshare_layers.cc b/webrtc/modules/video_coding/codecs/vp8/screenshare_layers.cc
index 536587a13ee07795f7bacab1c2eec4fbe41ffa27..282268a087eb0afeb6e4868e149bd68337ad44b4 100644
--- a/webrtc/modules/video_coding/codecs/vp8/screenshare_layers.cc
+++ b/webrtc/modules/video_coding/codecs/vp8/screenshare_layers.cc
@@ -147,7 +147,8 @@ bool ScreenshareLayers::ConfigureBitrates(int bitrate_kbps,
     }
 
     // Don't reconfigure qp limits during quality boost frames.
-    if (layers_[active_layer_].state != TemporalLayer::State::kQualityBoost) {
+    if (active_layer_ != -1 &&

[pbos-webrtc, 2016/02/02 21:16:14]

Can you amend the comment above to include what -1 as active_layer_ means in
this function?

[sprang, 2016/02/03 10:05:37]

Done.

+        layers_[active_layer_].state != TemporalLayer::State::kQualityBoost) {
       min_qp_ = cfg->rc_min_quantizer;
       max_qp_ = cfg->rc_max_quantizer;
       // After a dropped frame, a frame with max qp will be encoded and the
@@ -169,6 +170,7 @@ bool ScreenshareLayers::ConfigureBitrates(int bitrate_kbps,
 void ScreenshareLayers::FrameEncoded(unsigned int size,
                                      uint32_t timestamp,
                                      int qp) {
+  RTC_DCHECK_NE(-1, active_layer_);
   if (size == 0) {
     layers_[active_layer_].state = TemporalLayer::State::kDropped;
     return;
@@ -198,6 +200,7 @@ void ScreenshareLayers::PopulateCodecSpecific(bool base_layer_sync,
     vp8_info->layerSync = false;
     vp8_info->tl0PicIdx = kNoTl0PicIdx;
   } else {
+    RTC_DCHECK_NE(-1, active_layer_);
     vp8_info->temporalIdx = active_layer_;
     if (base_layer_sync) {
       vp8_info->temporalIdx = 0;
@@ -248,6 +251,7 @@ bool ScreenshareLayers::TimeToSync(int64_t timestamp) const {
 bool ScreenshareLayers::UpdateConfiguration(vpx_codec_enc_cfg_t* cfg) {
   if (max_qp_ == -1 || number_of_temporal_layers_ <= 1)
     return false;
+  RTC_DCHECK_NE(-1, active_layer_);
 
   // If layer is in the quality boost state (following a dropped frame), update
   // the configuration with the adjusted (lower) qp and set the state back to