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Cleanup: document that `MEM_dupallocN` is NULL-safe

Add comment explaining `MEM_dupallocN` is NULL-safe, in that it returns
NULL when it receives a NULL pointer. This is currently true for both
implementations of the function (`MEM_lockfree_dupallocN` and
`MEM_guarded_dupallocN`), and will be expected of other implementations
as well.

No functional changes.