Proof paths can be zero-length

[richsalz]

Ben Kaduk pointed out that the ConsistencyProofDataV2 and InclusionProofDataV2 had arrays that could not be empty. This was contradicted elsewhere in the document, so fix the 1.. to 0.. in both cases.

This should address the last part of Ben's DISCUSS.

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[robstradling]

The algorithm in Section 2.1.4.2 (Verifying Consistency between Two Tree Heads) requires at least 1 element in the consistency_path:
4. Set both "fr" and "sr" to the first value in the "consistency_path" array.

This section also says where "0 < first < second", which means that at least 1 element must be present for the consistency_path to be valid (because first != second). However, we can't assume that the consistency_path is valid until it has been validated.

So I suggest adding a new first step to the algorithm, with the following description:
If consistency_path is an empty array, stop the iteration and fail the proof verification.

WDYT?

[richsalz]

If consistency_path is an empty array, stop the iteration and fail the proof verification.

looks good; updated commit pushed.

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