Tidy up the code.
* mi/mi-cmd-var.c (mi_cmd_var_update): Remove variable nv. Move
variables var, rootlist and cr to their local sub-block. Move the
make_cleanup_ui_out_tuple_begin_end, make_cleanup_ui_out_list_begin_end
and do_cleanups calls to the common base block. Remove the zero NV
check.