Fixing browser crash in the linux tab dragging code.

chrome/browser/ui/gtk/tabs/dragged_tab_controller_gtk.cc

 // Copyright (c) 2011 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 "chrome/browser/ui/gtk/tabs/dragged_tab_controller_gtk.h"
 
 #include <algorithm>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
@@ skipping 68 matching lines @@
   drag_data_.reset();
 }
 
 void DraggedTabControllerGtk::CaptureDragInfo(const gfx::Point& mouse_offset) {
   start_screen_point_ = gfx::Screen::GetCursorScreenPoint();
   mouse_offset_ = mouse_offset;
 }
 
 void DraggedTabControllerGtk::Drag() {
   if (!drag_data_->GetSourceTabData()->tab_ ||
-      !drag_data_->GetSourceTabContentsWrapper()) {
+      !drag_data_->GetSourceTabContentsWrapper() ||
+      !drag_data_->GetSourceTabContents()) {
     return;
   }
 
   bring_to_front_timer_.Stop();
 
   EnsureDraggedView();
 
   // Before we get to dragging anywhere, ensure that we consider ourselves
   // attached to the source tabstrip.
   if (drag_data_->GetSourceTabData()->tab_->IsVisible()) {
@@ skipping 783 matching lines @@
 
 bool DraggedTabControllerGtk::AreTabsConsecutive() {
   for (size_t i = 1; i < drag_data_->size(); ++i) {
     if (drag_data_->get(i - 1)->source_model_index_ + 1 !=
         drag_data_->get(i)->source_model_index_) {
       return false;
     }
   }
   return true;
 }