GPU: Fix NULL gpu::Buffer dereference.

gpu/command_buffer/client/transfer_buffer.cc

Index: gpu/command_buffer/client/transfer_buffer.cc
diff --git a/gpu/command_buffer/client/transfer_buffer.cc b/gpu/command_buffer/client/transfer_buffer.cc
index da3996beeb9006d943e357d7357252d2171341f2..22a31c8c1ac19bd2a2129eb0f89e4c6f83576109 100644
--- a/gpu/command_buffer/client/transfer_buffer.cc
+++ b/gpu/command_buffer/client/transfer_buffer.cc
@@ -67,6 +67,7 @@ void TransferBuffer::Free() {
 }
 
 bool TransferBuffer::HaveBuffer() const {
+  DCHECK(buffer_id_ == -1 || buffer_);
   return buffer_id_ != -1;
 }
 
@@ -88,6 +89,7 @@ void TransferBuffer::AllocateRingBuffer(unsigned int size) {
     scoped_refptr<gpu::Buffer> buffer =
         helper_->command_buffer()->CreateTransferBuffer(size, &id);
     if (id != -1) {
+      DCHECK(buffer);
       buffer_ = buffer;
       ring_buffer_.reset(new AlignedRingBuffer(
           alignment_,