Added speculative fix for invalid memory access when app is in background

[wemstad]

This is a speculative fix for solving the issue regarding the invalid memory in #2

Theory

When the App is in the background at least 2 events get logged with the completionBlock in PeachCollectorQueue.m:

        dispatch_async(dispatch_get_main_queue(), ^{
            if ([[UIApplication sharedApplication] applicationState] != UIApplicationStateActive
                && [addedEvent shouldBeFlushedWhenReceivedInBackgroundState]) {
                [self flush];
            }
        
            [self checkPublishers];
        });

this codeblock then gets added to the queue multiple times, same as the number of events.
When the first queue later gets to the first event everything works. However, somewhere in the flush or checkPublishers call the second event get processed and deleted.
When the queue then gets to processing the second event the addedEvent object is nil, causing the shouldBeFlushedWhenReceivedInBackgroundState to crash.

Haven't been able to replicate this, therefore its purely speculative but I think it's worth trying to see if this solves our crashes