Fixed avoiding of other printed objects

Calling std::move on itself causes that the first polygon is empty, which results in disabling this feature

src/libslic3r/GCode.cpp

@@ -158,7 +158,7 @@ namespace Slic3r {
                             polygons_per_layer[i * 2] = union_(polys);
                         }
                     });
-                for (size_t i = 0; i < cnt / 2; ++i)
+                for (size_t i = 1; i < cnt / 2; ++i)
                     polygons_per_layer[i] = std::move(polygons_per_layer[i * 2]);
                 if (cnt & 1)
                     polygons_per_layer[cnt / 2] = std::move(polygons_per_layer[cnt - 1]);