Fix for random Springboard crashes

As discussed in #5, v1.1.3 of Priority Hub causes random Springboard crashes due to an `unrecognized selector` exception being thrown when `numNotificationsForAppID` and `- (double)tableView:(UITableView*)tableView heightForRowAtIndexPath:(NSIndexPath*)indexPath` in [Tweak.xm](https://github.com/thomasfinch/Priority-Hub/blob/master/Tweak.xm) come into contact with an object that is not of the `SBAwayBulletinListItem` class and ask it for the `activeBulletin` property. I have added a check for that class in both methods, which, for two days, has fixed the random crashing issues that me and a few friends had previously experienced.