Fix null dereference in GLES2Implementation::IsExtensionAvailable.

gpu/command_buffer/client/gles2_implementation.cc

 // Copyright (c) 2012 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.
 
 // A class to emulate GLES2 over command buffers.
 
 #include "../client/gles2_implementation.h"
 
 #include <algorithm>
 #include <map>
@@ skipping 247 matching lines @@
 }
 
 void GLES2Implementation::WaitForCmd() {
   TRACE_EVENT0("gpu", "GLES2::WaitForCmd");
   helper_->CommandBufferHelper::Finish();
 }
 
 bool GLES2Implementation::IsExtensionAvailable(const char* ext) {
   const char* extensions =
       reinterpret_cast<const char*>(GetStringHelper(GL_EXTENSIONS));
+  if (!extensions)
+    return false;
+
   int length = strlen(ext);
   while (true) {
     int n = strcspn(extensions, " ");
     if (n == length && 0 == strncmp(ext, extensions, length)) {
       return true;
     }
     if ('\0' == extensions[n]) {
       return false;
     }
     extensions += n + 1;
@@ skipping 3280 matching lines @@
   return helper_->InsertSyncPointCHROMIUM();
 }
 
 // Include the auto-generated part of this file. We split this because it means
 // we can easily edit the non-auto generated parts right here in this file
 // instead of having to edit some template or the code generator.
 #include "../client/gles2_implementation_impl_autogen.h"
 
 }  // namespace gles2
 }  // namespace gpu