last line of leading multiline message remains in cleaned stack

```js
// in REPL
const StackUtils = require('stack-utils');
const stack = new StackUtils();
try {
  throw new TypeError(`foo
bar
baz`);
} catch (err) {
  console.log(stack.clean(err.stack));
}
```

actual:

```
baz
    REPL18:2:9
```

expected:

either the entire message or none of it
