47.6: Harden reader validation for malformed input #16

Job	Run time
test-macos	3m 7s
cross-compile (aarch64-macos-none)	3m 5s
test-linux	2m 56s
cross-compile (x86_64-macos-none)	2m 55s
cross-compile (x86_64-linux-gnu)	2m 52s
cross-compile (aarch64-linux-gnu)	3m 1s
	17m 56s

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