Added extra DCHECK() to eliminate possible NULL deref.

CID=104002
BUG=
TEST=
TBR=ben

Review URL: https://chromiumcodereview.appspot.com/10822022

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@148434 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/ui/views/tabs/tab_drag_controller.cc

@@ -1610,7 +1610,7 @@ void TabDragController::ResetSelection(TabStripModel* model) {
 }
 
 void TabDragController::RevertDragAt(size_t drag_index) {
-  DCHECK(started_drag_);
+  DCHECK(started_drag_ && source_tabstrip_);
 
   TabDragData* data = &(drag_data_[drag_index]);
   if (attached_tabstrip_) {