Correct conversion of unscaled_duration to a uint64_t when unscaled_duration is 0x1p64.
Correct conversion of unscaled_duration to a uint64_t when unscaled_duration is 0x1p64.

It'd be clearer to check |>= 0x1p64| instead of |>= (double) UINT64_MAX|
(which behaves differently under different rounding modes), but that
requires C99 functionality only recently added to MSVC.

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