Fix issue 1527

https://github.com/minetest/minetest/issues/1527

src/script/lua_api/l_vmanip.cpp

@@ -31,8 +31,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 int LuaVoxelManip::gc_object(lua_State *L)
 {
 	LuaVoxelManip *o = *(LuaVoxelManip **)(lua_touserdata(L, 1));
-	if (!o->is_mapgen_vm)
+	delete o;
-		delete o;
 
 	return 0;
 }
@@ -319,7 +318,8 @@ LuaVoxelManip::LuaVoxelManip(Map *map)
 
 LuaVoxelManip::~LuaVoxelManip()
 {
-	delete vm;
+	if (!is_mapgen_vm)
+		delete vm;
 }
 
 // LuaVoxelManip()