Fix bug in verifyProof

[no2chem]

This PR fixes a bug in verify proof where "Error: Unexpected end of proof" if the tree contains an extension node with an embedded branch node.

A simple example of this bug is below:

const Tree = require('merkle-patricia-tree');
const tree = new Tree();
tree.put('a', 'a', (err) => {
    tree.put('b', 'b', (err) => {
         Tree.prove(tree, 'a', (err, proof) => {
            Tree.verifyProof(tree.root, 'a', proof, (err, val) => {
                // err = "Error: Unexpected end of proof"
                // expected : val === 'a' 
            }
        } 
    }
}

Fixes #52.

[coveralls]

Coverage decreased (-0.3%) to 80.791% when pulling ceefb10 on no2chem:verifyBranchFix into bc42cde on ethereumjs:master.

[no2chem]

It seems that the coveralls configuration for this project doesn't reflect proof.js.

This is probably because L8 from package.json only runs coverage for ./test/index.js.

[holgerd77]

Ok, this looks good, will approve now, @no2chem thanks again for the PR.

Do you depend on this for another library and need this as a release or does this has some time and can be released in a combined release a bit later?

[no2chem]

@holgerd77 Thanks! No, we don't have a particular need for it right now, so it can be combined as a release sometime later.