Account for missing NSFileProtectionKey file attribute key.

Ok, after doing a bit of testing, it appears that the NSFileProtectedKey is not included in the file attributes dictionary while running on the iOS simulator. It *does* get returned on a device, however. I tested on iOS 7.0.6.

When the key is missing, it forces a new log file to always be created, due to the test `if (! [key isEqualToString:NSFileProtectionCompleteUntilFirstUserAuthentication]) {` near line ~946.

Therefore, this patch will assume that we can reuse the existing log file if the NSFileProtectedKey is missing. If you're not comfortable with this, we could add checks for #if TARGET_IPHONE_SIMULATOR to fine-tune this condition.

Thanks!