Ignore output callbacks with null parameters in iOS H264 encode.

This can occur if there are callbacks in-flight when the compression
session is destroyed. Has been observed but is rare.

Bug: webrtc:8489
Change-Id: I5d4b35c555f6ff68af48edfcc7acf53395fa86fe
Reviewed-on: https://webrtc-review.googlesource.com/18220
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20561}

sdk/objc/Framework/Classes/VideoToolbox/RTCVideoEncoderH264.mm

@@ -151,10 +151,12 @@ void compressionOutputCallback(void *encoder,
                                OSStatus status,
                                VTEncodeInfoFlags infoFlags,
                                CMSampleBufferRef sampleBuffer) {
-  RTC_CHECK(params);
+  if (!params) {
+    // If there are pending callbacks when the encoder is destroyed, this can happen.
+    return;
+  }
   std::unique_ptr<RTCFrameEncodeParams> encodeParams(
       reinterpret_cast<RTCFrameEncodeParams *>(params));
-  RTC_CHECK(encodeParams->encoder);
   [encodeParams->encoder frameWasEncoded:status
                                    flags:infoFlags
                             sampleBuffer:sampleBuffer