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/Classes/PeerConnection/RTCRtpReceiver.mm b/sdk/objc/Framework/Classes/PeerConnection/RTCRtpReceiver.mm
index 0888f24..cb2771a 100644
--- a/sdk/objc/Framework/Classes/PeerConnection/RTCRtpReceiver.mm
+++ b/sdk/objc/Framework/Classes/PeerConnection/RTCRtpReceiver.mm
@@ -58,7 +58,7 @@
   }
 }
 
-- (RTCMediaStreamTrack *)track {
+- (nullable RTCMediaStreamTrack *)track {
   rtc::scoped_refptr<webrtc::MediaStreamTrackInterface> nativeTrack(
     _nativeRtpReceiver->track());
   if (nativeTrack) {