Verify Proof fails if there is an extension with an embedded branch

[no2chem]

This can occur if you insert lots of small key/values into the tree.

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' 
            }
        } 
    }
}

PR #51 fixes this issue.