FIX: 'account_name' referenced before assignment

resolve error :
File "/usr/local/shinken/shinken/modules/webui_broker/webui_broker.py",
line 525, in check_auth
r = f(user, password)
File "/usr/local/shinken/shinken/modules/active_directory_ui.py", line
249, in check_auth
local_con.simple_bind_s(account_name, password)
UnboundLocalError: local variable 'account_name' referenced before
assignment