Prevent error when trying to move a configurable product in incorrect attset

... in the case one or more configurable attributes are not found in
the target attribute set

Prevents the following unhandled error:
```
Fatal error: Call to a member function getAttributeCode() on a non-object in /path/to/htdocs/app/code/core/Mage/Catalog/Model/Product/Type/Configurable.php on line 404
Call Stack
#    Time    Memory    Function    Location
1    0.0001    658024    {main}( )    ../index.php:0
2    0.0027    1240064    Mage::run( )    ../index.php:87
3    0.0105    2819192    Mage_Core_Model_App->run( )    ../Mage.php:684
4    0.4158    15318784    Mage_Core_Controller_Varien_Front->dispatch( )    ../App.php:354
5    0.4186    15759136    Mage_Core_Controller_Varien_Router_Standard->match( )    ../Front.php:172
6    0.4225    16679400    Mage_Core_Controller_Varien_Action->dispatch( )    ../Standard.php:250
7    0.5588    24536792    Flagbit_ChangeAttributeSet_Adminhtml_Catalog_ProductController->changeattributesetAction( )    ../Action.php:418
8    0.5764    30512936    Mage_Eav_Model_Entity_Collection_Abstract->save( )    ../ProductController.php:44
9    0.5764    30512936    Mage_Core_Model_Abstract->save( )    ../Abstract.php:944
10    0.5766    30513056    Mage_Catalog_Model_Product->_beforeSave( )    ../Abstract.php:316
11    0.6224    31500280    Mage_Catalog_Model_Product_Type_Configurable->beforeSave( )    ../Product.php:470```