In conf_apply_rules, check input argument is a table before assuming it
is and potentially crashing.
Add check also in wplua_table_to_properties to avoid similar bugs.
7908b8d7be2a2992c57cd549054eda7ce46e4b44 ("m-lua-scripting: allow
converting GValue holding NULL objects to Lua") accidentally added a second
refcount. As a result, the objects are never freeded.
Remove the second refcount to fix this.