Fix leak in var arg handling

When using the mixed position + vararg path, pybind over inc_ref's
the vararg positions. Printing the ref_count() of `item` before
and after this change you see:

Before change:

```
refcount of item before assign 3
refcount of item after assign 5
```

After change
```
refcount of item before assign 3
refcount of item after assign 4
```