sp-trie: minor fix to avoid possible panic during node decoding #5973

Job	Run time
preflight / preflight	32s
preflight / ci-versions	1m 36s
test-frame-ui	49m 6s
cargo-check-each-crate (1)	26m 39s
test-full-crypto-feature	2m 40s
cargo-check-benches (master)	21m 22s
cargo-check-each-crate (2)	25m 29s
cargo-check-benches (current)	20m 56s
cargo-check-each-crate (3)	9m 53s
cargo-check-each-crate (4)	30m 17s
cargo-check-all-crate-macos	11m 45s
cargo-check-each-crate (5)	37m 21s
test-node-metrics	8m 34s
cargo-check-each-crate (6)	15m 6s
cargo-check-each-crate (7)	38m 28s
test-frame-examples-compile-to-wasm	3m 11s
test-deterministic-wasm	6m 48s
check-tracing	2m 26s
check-metadata-hash	6m 39s
node-bench-regression-guard	12s
All test misc tests passed	1s
	5h 19m 1s

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