Fix narrowing conversion warning in struct fstring #7618

Job	Run time
build (windows-2019, Win32, v142, 17, Debug)	1m 10s
build (windows-2019, Win32, v142, 17, Release)	2m 18s
mingw (mingw64)	3m 28s
mingw (ucrt64)	4m 9s
build (windows-2019, Win32, v142, 17, -DBUILD_SHARED_LIBS=ON, Debug)	1m 18s
build (windows-2019, Win32, v142, 17, -DBUILD_SHARED_LIBS=ON, Release)	2m 6s
build (windows-2019, x64, v141, 14, Debug)	1m 48s
build (windows-2019, x64, v141, 14, Release)	3m 9s
build (windows-2019, x64, v141, 17, Debug)	1m 17s
build (windows-2019, x64, v141, 17, Release)	3m 5s
build (windows-2019, x64, v142, 17, Debug)	1m 24s
build (windows-2019, x64, v142, 17, Release)	2m 9s
build (windows-2019, x64, v142, 17, -DBUILD_SHARED_LIBS=ON, Debug)	1m 38s
build (windows-2019, x64, v142, 17, -DBUILD_SHARED_LIBS=ON, Release)	2m 6s
build (windows-2019, x64, v142, 20, Debug)	2m 4s
build (windows-2019, x64, v142, 20, Release)	2m 50s
build (windows-2022, x64, v143, Debug, 20)	2m 24s
	38m 23s

Provide feedback