cros: Fix crash in about:tracing

chromeos/dbus/debug_daemon_client.cc

Index: chromeos/dbus/debug_daemon_client.cc
diff --git a/chromeos/dbus/debug_daemon_client.cc b/chromeos/dbus/debug_daemon_client.cc
index 23813a846ca69e890e0e2d484933f4fab260b68b..05d3b394ddce75695455ce808f9ccd1209c04f3f 100644
--- a/chromeos/dbus/debug_daemon_client.cc
+++ b/chromeos/dbus/debug_daemon_client.cc
@@ -470,7 +470,10 @@ class DebugDaemonClientImpl : public DebugDaemonClient {
   void OnRequestStopSystemTracing(dbus::Response* response) {
     if (!response) {
       LOG(ERROR) << "Failed to request systrace stop";
-      pipe_reader_->OnDataReady(-1); // terminate data stream
+      // If debugd crashes or completes I/O before this message is processed
+      // then pipe_reader_ can be NULL, see OnIOComplete().
+      if (pipe_reader_.get())
+        pipe_reader_->OnDataReady(-1); // terminate data stream
     }
     // NB: requester is signaled when i/o completes
   }