The buffer `buf` contains N (= `slice_sizes.len`) slices followed by the
N null-terminated arguments. The N null-terminated arguments are stored
in the `contents` array list. Thus, `buf` size should be:
    @sizeOf([]u8) * slice_sizes.len + contents_slice.len

Instead of:
    @sizeOf([]u8) * slice_sizes.len + contents_slice.len + slice_sizes.len

This bug was found thanks to the gpa allocator which checks if freed
size matches allocated sizes for large allocations.