mi_xmlrpc: fix crash when flushing complex MI trees as strings

	* crash was replicated with "ul_dump" command at 51+ contacts
		- "reply_option" must be disabled

This patch solves a couple of other issues:
	* properly mark completed nodes when "reply_option" = 0
	* a rare memory leak when flushing empty "mi_node"s
(cherry picked from commit f5da89d27099f405e1242347bdf9e3cb6368423c)

Conflicts:

	modules/mi_xmlrpc/xr_writer.c