worker: Fix memory corruption error/crasher
    
    gdm_session_worker_process_pam_message() contains this code:
    
                             *response_text = strndup (user_answer,
    PAM_MAX_RESP_SIZE - 1);
                            (*response_text)[PAM_MAX_RESP_SIZE - 1] = '\0';
    
    If the string pointed to by user_answer is shorter than PAM_MAX_RESP_SIZE - 1
    (which will generally be the case), the second line clobbers unrelated memory.
    On this powerpc laptop, that causes gdm-session-worker to crash while verifying
    the password, leaving me unable to log into any user session.
    
    strndup() already ensures that the resulting string is 0-terminated anyway, so
    this commit just removes the second line.