Fixes narrowing conversion. #491

Job	Run time
B2 msvc-14.2	5m 20s
CMake msvc-14.2 Debug C++17	17m 17s
B2 msvc-14.3	5m 58s
CMake gcc-11 17 Debug	8m 51s
CMake msvc-14.2 Release C++17	7m 46s
CMake gcc-11 20 Release	11m 2s
CMake msvc-14.3 Debug C++20	10m 17s
CMake clang-11 17 Debug	6m 9s
CMake msvc-14.3 Release C++20	9m 56s
CMake clang-11 20 Debug	6m 36s
CMake clang-13 17 Release	9m 16s
CMake clang-13 20 Release	9m 15s
B2 gcc-11	3m 37s
CMake clang-14 17 Debug -stdlib=libc++	8m 27s
B2 clang-14	3m 37s
CMake clang-14 20 Release -stdlib=libc++	11m 40s
	2h 15m 4s

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