fix end proof verification

[danlaine]

Why this should be merged

The invariant for RangeProof's EndProof field says:

// If no upper range bound was given, [KeyValues] is empty,
// and [StartProof] is non-empty, this is empty.
//
// If no upper range bound was given, [KeyValues] is empty,
// and [StartProof] is empty, this is the root.
//
// If an upper range bound was given and [KeyValues] is empty,
// this is a proof for the upper range bound.
//
// Otherwise, this is a proof for the largest key in [KeyValues].
EndProof []ProofNode

But in RangeProof.Verify we check:

case len(proof.EndProof) == 0 && len(end) > 0:
	return ErrNoEndProof

This should be:

case len(end) > 0 && len(proof.KeyValues) == 0 && len(proof.EndProof) == 0:
	return ErrNoEndProof

i.e. the invariant says there should be an end proof iff all of [start proof, key values, upper bound] are empty but in the check we only look at two of those three (namely, we weren't checking the key values)

How this works

See above

How this was tested

HyperSDK CI