idrange: fix unassigned global variable
    
    Global variable '_dcerpc_bindings_installed' is in some cases used
    before assigment. This patch ensures that _dcerpc_bindings_installed is
    always initialized.
    
    https://fedorahosted.org/freeipa/ticket/6082
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>