bugfix: invalid variable reference

variable v does not exist... the function is not commented so I
can't be sure of the intention, but the most backwards-compatible
fix is to not pass argument v, so that it will be nil as before.

this bug was revealed by enabling lua "strict mode"

data/lua/location_set.lua

@@ -100,7 +100,7 @@ function methods:stable_iter(f)
 	table.sort(indices)
 	for i,p in ipairs(indices) do
 		local x, y = revindex(p)
-		f(x, y, v)
+		f(x, y)
 	end
 end