Fix addPendingEvent with pending event order == chronological
When the pending event order setting was set to 'chronological'
(the default) `addPendingEvent` would NPE because Room no longer
has a `this._filter` property. It should get the filter from the
event timeline set instead, as it does in the previous line when
checking or the presence of a filter.
We should strongly consider changing the default pending event order
to 'detached' and probably removing 'chronological' or comitting to
support it properly: it's not really tested and is prone to breakage
like this.
Applies flumpt's fix from https://github.com/matrix-org/matrix-js-sdk/issues/599
Fixes https://github.com/matrix-org/matrix-js-sdk/issues/599