[krbdev.mit.edu #6903] Fix memory leak in kpasswd server UDP error path

[Greg Hudson via RT]

Although it's totally non-obvious, there is no memory leak after #6899.  
When rep->data is allocated in process_chpw_request(), ret should always 
be 0 (this is the non-obvious part).  So when a non-zero return happens, 
ret->data will be NULL, and the new code in #6899 will free the container.