The previous code would fail to return next_batch if any of the events
in the window were not visible to the user. It would also return an
unnecessary next_batch when no more results are available if the total
number of results is exactly `skip + limit`.
This bug will become much more severe with a full filtering
implementation, because we will be more likely to trigger it by
filtering out events in a search call. Currently, it is only possible to
trigger with rooms that have history visibility set to "invited" or
"joined".
Signed-off-by: strawberry <strawberry@puppygock.gay>