Avoid left shift of negative values when parsing H264 stream

media/filters/h264_bit_reader.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/logging.h"
 #include "media/filters/h264_bit_reader.h"
 
 namespace media {
 
 H264BitReader::H264BitReader()
@@ skipping 38 matching lines @@
 
     if (bytes_left_ < 1)
       return false;
   }
 
   // Load a new byte and advance pointers.
   curr_byte_ = *data_++ & 0xff;
   --bytes_left_;
   num_remaining_bits_in_curr_byte_ = 8;
 
-  prev_two_bytes_ = (prev_two_bytes_ << 8) | curr_byte_;
+  prev_two_bytes_ = ((prev_two_bytes_ & 0xff) << 8) | curr_byte_;
 
   return true;
 }
 
 // Read |num_bits| (1 to 31 inclusive) from the stream and return them
 // in |out|, with first bit in the stream as MSB in |out| at position
 // (|num_bits| - 1).
 bool H264BitReader::ReadBits(int num_bits, int* out) {
   int bits_left = num_bits;
   *out = 0;
@@ skipping 43 matching lines @@
 
   bytes_left_ = 0;
   return false;
 }
 
 size_t H264BitReader::NumEmulationPreventionBytesRead() {
   return emulation_prevention_bytes_;
 }
 
 }  // namespace media