Fixes C4838 warnings:

  rfc6234/sha384-512.c(326): warning C4838: conversion from '__int64' to 'quint64' requires a narrowing conversion
  rfc6234/sha384-512.c(331): warning C4838: conversion from '__int64' to 'quint64' requires a narrowing conversion
  rfc6234/sha384-512.c(796): warning C4838: conversion from '__int64' to 'const quint64' requires a narrowing conversion

Change-Id: Ib531d376d3fd49785e227455f280fc4072bba8c5
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>