Fix calculation of aligned buffer size

As is, for requested size which are already aligned, we over-allocate,
so we fix this. We also fix the allocation for chunk size 1.

This

Fix calculation of aligned buffer size

As is, for requested size which are already aligned, we over-allocate,
so we fix this. We also fix the allocation for chunk size 1.

This issue has been reported by @kkmuffme.

Thanks to @iluuu1994 for improving the fix!

Closes GH-16161.

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