sp-trie: minor fix to avoid possible panic during node decoding #13806

Job	Run time
check-dependency-rules	11s
check-fail-ci	11s
check-markdown	17s
check-workspace	19s
preflight / preflight	49s
preflight / ci-versions	1m 19s
check-toml-format	1m 9s
fmt	1m 33s
check-rust-feature-propagation	2m 15s
check-umbrella	2m 40s
test-rust-features	2m 33s
All quick checks passed	1s
	13m 17s

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