fix crash when multiple targets depend on the same target

moonbuild/core/DAG.moon

@@ -34,6 +34,7 @@ transclosure = (obj, prop) ->
 	i = 1
 	set = {}
 	imp = (e) ->
+		return unless e[prop]
 		for v in *e[prop]
 			if not set[v]
 				elems[i], i = v, i+1

out/moonbuild

@@ -1695,6 +1695,9 @@ transclosure = function(obj, prop)
   local set = { }
   local imp
   imp = function(e)
+    if not (e[prop]) then
+      return 
+    end
     local _list_0 = e[prop]
     for _index_0 = 1, #_list_0 do
       local v = _list_0[_index_0]