verifyProof does not throw an error on invalid proofs

[brickpop]

We are using the BaseTrie class to check the validity of Ethereum storage proofs and proof of non-existence.

According to the Readme, verifyProof should be throwing an error if proof is found to be invalid.

However:

• We are setting invalid values to the proof array but no error is thrown
• It is thrown is the root is the invalid value (expected)

By always receiving null as a response, we have no way to tell if it is because (1) the proof is valid and there is no such value or because (2) the proof is invalid and the value doesn't match anyway.

Steps to reproduce:

• Adding the following test case to trie/test/index.spec.ts

tape('shall throw an error when a proof is invalid', async (t) => {
  const trie = new BaseTrie()
  await trie.put(Buffer.from('a'), Buffer.from('value1'))
  await trie.put(Buffer.from('aa'), Buffer.from('value2'))
  await trie.put(Buffer.from('aaa'), Buffer.from('value3'))

  const proof = await BaseTrie.createProof(trie, Buffer.from("a"))

  // Normal verification
  await BaseTrie.verifyProof(trie.root, Buffer.from('a'), proof)

  // Corrupting the proof
  proof[0].reverse()

  try {
    // @throws — If proof is found to be invalid.
    await BaseTrie.verifyProof(trie.root, Buffer.from('a'), proof)
    t.fail('should have thrown an error, but didn\'t')
  } catch (err) { }

  t.end()
})

Some testing for invalid roots should also be added.
Thank you!

(thanks @ed255 for the discovery)