Check getTransition(old) return value

This works around

    TypeError: Cannot read property 'teardown' of undefined
    at core-overlay.Polymer.transitionChanged (http://jeffposnick.github.io/pageable-text/components/core-overlay/core-overlay.html:312:32)

runtime exceptions that I see in [this demo](http://jeffposnick.github.io/pageable-text/components/pageable-text/demo.html).

I'm not using `<core-overlay>` directly, and I assume the errors come from using `<paper-dialog>`. The value of `old` is `"core-transition-fade"` when this exception is thrown.