make to_float(level) handle invalid #160

Job	Run time
formatting	48s
linux (linux-amd64-release, linux-amd64)	2m 59s
msvc (Debug)	6m 7s
linux (linux-arm64-release, linux-arm64, qemu-aarch64-static)	2m 12s
macos (macos-x86_64, macos-13)	3m 54s
msvc (Release)	7m 28s
linux (clang-tidy)	5m 23s
macos (macos-arm64, macos-14)	1m 32s
linux (linux-sanitizer)	2m 15s
linux (linux-debug, valgrind --leak-check=full --error-exitcode=1)	4m 35s
docker	20s
	37m 33s

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