Fix narrowing conversion warning in struct fstring #7611

Job	Run time
build (g++-4.9, Debug, 11)	1m 49s
build (g++-4.9, Release, 11)	2m 33s
build (g++-10, Debug, 11)	1m 39s
build (g++-10, Release, 11)	2m 9s
build (clang++-9, Debug, 11)	1m 25s
build (clang++-9, Release, 11)	2m 13s
build (g++-8, Debug, 14, sudo apt install g++-8)	1m 28s
build (g++-8, Debug, 17, sudo apt install g++-8)	1m 29s
build (g++-9, Debug, 17)	1m 35s
build (g++-10, Debug, 17)	1m 25s
build (g++-11, Debug, 20, sudo apt install g++-11)	2m 9s
build (clang++-8, Debug, 17, -stdlib=libc++, sudo apt install clang-8 libc++-8-dev libc++abi-8-dev)	1m 31s
build (clang++-9, Debug, 17, -DFMT_FUZZ=ON -DFMT_FUZZ_LINKMAIN=ON, sudo apt install clang-9)	1m 58s
build (clang++-11, Debug, 20)	1m 45s
build (clang++-11, Debug, 20, -stdlib=libc++, sudo apt install libc++-11-dev libc++abi-11-dev)	1m 49s
build (g++-13, Release, 23, -DBUILD_SHARED_LIBS=ON, sudo apt install g++-13)	3m 42s
	30m 39s

Provide feedback