Fixes narrowing conversion. #493

Job	Run time
B2 msvc-14.2	4m 50s
B2 gcc-11	3m 31s
B2 msvc-14.3	5m 27s
CMake msvc-14.2 Debug C++17	11m 37s
B2 clang-14	3m 47s
CMake gcc-11 17 Debug	8m 40s
CMake msvc-14.2 Release C++17	8m 24s
CMake gcc-11 20 Release	10m 40s
CMake msvc-14.3 Debug C++20	8m 44s
CMake clang-11 17 Debug	6m 7s
CMake msvc-14.3 Release C++20	9m 28s
CMake clang-11 20 Debug	6m 25s
CMake clang-13 17 Release	8m 59s
CMake clang-13 20 Release	9m 22s
CMake clang-14 17 Debug -stdlib=libc++	8m 16s
CMake clang-14 20 Release -stdlib=libc++	11m 20s
	2h 5m 37s

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