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.
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