[webgl-point] Add an assertion to avoid compile error

This adds a `goog.asserts.fail()` to `ol.style.AtlasManager#add` to avoid an error at compilation time:

```
2014-11-07 16:51:14,181 build/ol.js: node tasks/build.js config/ol.json build/ol.js
info ol Parsing dependencies
info ol Compiling 347 sources
ERR! compile /home/elemoine/src/ol3/src/ol/style/atlasmanager.js:107: ERROR - Missing return statement. Function expected to return (null|{image: (HTMLCanvasElement|null), offsetX: number, offsetY: number}).
ERR! compile     function(id, width, height, renderCallback, opt_this) {
ERR! compile     ^
ERR! compile
ERR! compile
ERR! compile 1 error(s), 0 warning(s),
ERR! compile 98.1% typed
ERR! compile
ERR! Process exited with non-zero status, see log for more detail: 1
2
```

@tsauerwein, I'm going to merge this because the compilation error is annoying. Please tell me if this is not a good fix for it.