Linux: don't crash when closing tabs that have open login prompts.

chrome/browser/login_prompt_gtk.cc

Index: chrome/browser/login_prompt_gtk.cc
===================================================================
--- chrome/browser/login_prompt_gtk.cc	(revision 26017)
+++ chrome/browser/login_prompt_gtk.cc	(working copy)
@@ -59,6 +59,8 @@
   }
 
   void SetModel(LoginModel* model) {
+    if (login_model_)
+      login_model_->SetObserver(NULL);
     login_model_ = model;
     if (login_model_)
       login_model_->SetObserver(this);
@@ -206,6 +208,8 @@
           this, &LoginHandlerGtk::SendNotifications));
     }
 
+    SetModel(NULL);
+
     // Delete this object once all InvokeLaters have been called.
     request_loop_->ReleaseSoon(FROM_HERE, this);
   }