Fixes narrowing conversion. #490

Job	Run time
B2 gcc-11	3m 46s
CMake gcc-11 17 Debug	8m 24s
B2 clang-14	3m 34s
CMake gcc-11 20 Release	11m 6s
CMake clang-11 17 Debug	6m 26s
CMake clang-11 20 Debug	7m 0s
B2 msvc-14.2	5m 14s
CMake clang-13 17 Release	9m 14s
CMake clang-13 20 Release	9m 32s
B2 msvc-14.3	6m 28s
CMake clang-14 17 Debug -stdlib=libc++	8m 48s
CMake clang-14 20 Release -stdlib=libc++	11m 56s
CMake msvc-14.2 Debug C++17	7m 28s
CMake msvc-14.2 Release C++17	9m 59s
CMake msvc-14.3 Debug C++20	10m 47s
CMake msvc-14.3 Release C++20	9m 3s
	2h 8m 45s

Provide feedback