Fix crash in BlitFramebufferCHROMIUM with a null read buffer.

gpu/command_buffer/service/gles2_cmd_decoder.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.
 
 #include "gpu/command_buffer/service/gles2_cmd_decoder.h"
 
 #include <limits.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
@@ skipping 7625 matching lines @@
         LOCAL_SET_GL_ERROR(GL_INVALID_OPERATION, func_name,
                            "incompatible src/dst color formats");
         return;
       }
       // Check whether draw buffers have identical color image with read buffer
       if (is_feedback_loop == FeedbackLoopUnknown) {
         GLenum attachment = static_cast<GLenum>(GL_COLOR_ATTACHMENT0 + ii);
         DCHECK(draw_framebuffer);
         const Framebuffer::Attachment* draw_buffer =
             draw_framebuffer->GetAttachment(attachment);
-        if (!draw_buffer) {
+        if (!draw_buffer || !read_buffer) {
           continue;
         }
         if (draw_buffer->IsSameAttachment(read_buffer)) {
           is_feedback_loop = FeedbackLoopTrue;
           break;
         }
       }
     }
   }
   if (is_feedback_loop == FeedbackLoopTrue) {
@@ skipping 10321 matching lines @@
 }
 
 // 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 "base/macros.h"
 #include "gpu/command_buffer/service/gles2_cmd_decoder_autogen.h"
 
 }  // namespace gles2
 }  // namespace gpu