fix _scratch 'leak' to global/window

[Xcrux]

_scratch was leaking because checkpointInterface was called without assigning this (in strict mode it would fail because this would be undefined, otherwise it takes global/window as 'this', hence the leak)
I'm not familiar with tape, if there's an option to check leaks (like --check-leaks in mocha) it would be great to use - it will never happen again in the future. I didn't find something similar to that in tape.

function checkpointInterface (trie) {
  this._scratch = null // before the fix `this` may be global, window or undefined
  // ...
}

[coveralls]

Coverage remained the same at 81.135% when pulling 2818d8a on Xcrux:master into 67fb885 on ethereumjs:master.

[holgerd77]

Thanks for this PR, I would have never seen this! Not sure about directly testing this with tape.