Fix SimpleBlock header struct parse error #258

Job	Run time
code-coverage	37s
clippy-rustfmt	35s
tests (stable-x86_64-unknown-linux-gnu)	35s
tests (stable-i686-pc-windows-msvc)	1m 29s
tests (stable-x86_64-pc-windows-msvc)	2m 12s
tests (stable-aarch64-apple-darwin)	45s
	6m 13s

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