[Chromecast] Fix crash in AudioLogMessage::Cancel()

Bug: internal b/160616014
Change-Id: If764ba82678059a21cda3d9ecd44c7dbe620fb92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2410926
Auto-Submit: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Yuchen Liu <yucliu@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806781}

chromecast/media/audio/audio_log.cc

@@ -188,7 +188,9 @@ AudioLogMessage::~AudioLogMessage() {
 }
 
 void AudioLogMessage::Cancel() {
-  buffer_->Cancel();
+  if (buffer_) {
+    buffer_->Cancel();
+  }
 }
 
 void InitializeAudioLog() {