i-bem__dom: afterCurrentEvent should guarantee block existence

[narqo]

It's a common case in our BEMDOM blocks to have a code like this:

// myblock.js
this.afterCurrentEvent(function() {
  this.domElem && this.doSomething();
});

We defiantly should make the kind of check like the one above as i-bem__dom functionality as we did with i-bem in bem-core library.

NOTE: this.domElem is the only true way to check if block still haven't been destructed (#507).

[deeonis]

BEM-2124