Fixing some of the issues found by clang static analyzer.

Bug: webrtc:8737
Change-Id: Ib436449c493336e7c35a72a96dc88cccdbb5bbaf
Reviewed-on: https://webrtc-review.googlesource.com/39200
Commit-Queue: Peter Hanspers <peterhanspers@webrtc.org>
Reviewed-by: Henrik Andreassson <henrika@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21607}
diff --git a/sdk/objc/Framework/Headers/WebRTC/RTCFileLogger.h b/sdk/objc/Framework/Headers/WebRTC/RTCFileLogger.h
index 5656b7b..1e8a591 100644
--- a/sdk/objc/Framework/Headers/WebRTC/RTCFileLogger.h
+++ b/sdk/objc/Framework/Headers/WebRTC/RTCFileLogger.h
@@ -69,7 +69,7 @@
 
 // Returns the current contents of the logs, or nil if start has been called
 // without a stop.
-- (NSData *)logData;
+- (nullable NSData *)logData;
 
 @end