Fix the issue of undefined-shift in VP8GetBit.

webrtc/modules/video_coding/utility/vp8_header_parser.cc

Index: webrtc/modules/video_coding/utility/vp8_header_parser.cc
diff --git a/webrtc/modules/video_coding/utility/vp8_header_parser.cc b/webrtc/modules/video_coding/utility/vp8_header_parser.cc
index 631385d0f25095903e676f79dc24e04ea1e1c6cd..1338e6ff47064c46842409d96aaf1adc0de0dd64 100644
--- a/webrtc/modules/video_coding/utility/vp8_header_parser.cc
+++ b/webrtc/modules/video_coding/utility/vp8_header_parser.cc
@@ -66,6 +66,7 @@ static void VP8InitBitReader(VP8BitReader* const br,
   br->value_ = 0;
   br->bits_ = -8;  // To load the very first 8bits.
   br->eof_ = 0;
+  br->error_ = false;

[pbos-webrtc, 2016/04/18 19:25:33]

Is this already covered by the eof_ flag? If so we don't need error_.

[jackychen_, 2016/04/18 20:49:47]

Not really, the error here might not be the same with what eof_ covers,
but do you want me to reuse eof_ to also cover this error?

[pbos-webrtc, 2016/04/18 20:52:02]

Have you diagnosed the actual error that happens?

   VP8LoadNewBytes(br);
 }
 
@@ -75,7 +76,10 @@ static int VP8GetBit(VP8BitReader* const br, int prob) {
   if (br->bits_ < 0) {
     VP8LoadNewBytes(br);
   }
-
+  if (br->bits_ < 0) {

[pbos-webrtc, 2016/04/18 19:25:33]

Shouldn't this be set inside VP8LoadNewBytes when it fails?

[jackychen_, 2016/04/18 20:49:47]

I want to stop the operation on br->bits here since otherwise I need to change
the return value of VP8LoadNewBytes and also add the judge here. Besides, the
error may also happens in VP8LoadFinalBytes. So I thought it might be better to
put it here.

+    br->error_ = true;
+    return 0;
+  }
   const int pos = br->bits_;
   const uint8_t split = (range * prob) >> 8;
   const uint8_t value = static_cast<uint8_t>(br->value_ >> pos);
@@ -195,6 +199,10 @@ bool GetQp(const uint8_t* buf, size_t length, int* qp) {
     LOG(LS_WARNING) << "Failed to get QP, end of file reached.";
     return false;
   }
+  if (br.error_) {
+    LOG(LS_WARNING) << "Failed to get QP, invalid bitstream.";

[pbos-webrtc, 2016/04/18 20:56:46]

Are you sure that this isn't because the stream is cut off (e.g. should go under
eof_?)?

+    return false;
+  }
   *qp = base_q0;
   return true;
 }