Fix for dangling session pointer in FBAppEvents

It seems that somewhere in the SDK an `FBSession` is being over released causing `lastSessionLoggedTo` to point to an invalid object. I've been able to reproduce a crash when the `flushTimer` is fired and running the code with zombies enabled will show `[FBSession retain] message sent to deallocated instance`. This fix works but someone who is more familiar with the code could probably figure out where the session is being released.