sp-trie: minor fix to avoid possible panic during node decoding (#6486)

# Description

This PR is a simple fix consisting of adding a check to the process of
decoding nodes of a storage proof to avoid panicking when receiving
badly-constructed proofs, returning an error instead.

This would close #6485

## Integration

No changes have to be done downstream, and as such the version bump
should be minor.

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Co-authored-by: Bastian Köcher <[email protected]>