Stop transition if element is outside fo the DOM

I have a backbone application, where map is rendered once and then stored globally. And sometimes when users open another page before animation is complete, it gives me a bunch of errors.

```
Uncaught TypeError: Cannot read property '1' of null leaflet.js:8210
L.PosAnimation.L.Class.extend._getPos leaflet.js:8210
L.PosAnimation.L.Class.extend._onStep leaflet.js:8193
(anonymous function)
```
on that line.

```javascript
_getPos: function () {
		var left, top, matches,
		    el = this._el,
		    style = window.getComputedStyle(el);

		if (L.Browser.any3d) {
			matches = style[L.DomUtil.TRANSFORM].match(this._transformRe);
			left = parseFloat(matches[1]);
--------------   Uncaught TypeError: Cannot read property '1' of null
			top  = parseFloat(matches[2]);
		} else {
			left = parseFloat(style.left);
			top  = parseFloat(style.top);
		}

		return new L.Point(left, top, true);
	},
```