powerd: do not crash at PrepareForSuspend if alarm is not supported.

BUG=chromium:1135901
TEST=CQ, power_WakeSources

Change-Id: Ia3d152dbd4a9a640e92a192cff2d78f146a30ba1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2477179
Tested-by: Qijiang Fan <fqj@google.com>
Reviewed-by: Qijiang Fan <fqj@google.com>
Commit-Queue: Qijiang Fan <fqj@google.com>
diff --git a/power_manager/powerd/system/suspend_configurator.cc b/power_manager/powerd/system/suspend_configurator.cc
index 9d2aa24..d41f3a8 100644
--- a/power_manager/powerd/system/suspend_configurator.cc
+++ b/power_manager/powerd/system/suspend_configurator.cc
@@ -66,6 +66,10 @@
 
   // Do this at the end so that system spends close to |suspend_duration| in
   // suspend.
+  if (!alarm_) {
+    LOG(ERROR) << "System doesn't support CLOCK_REALTIME_ALARM.";
+    return;
+  }
   if (suspend_duration != base::TimeDelta()) {
     alarm_->Start(FROM_HERE, suspend_duration, base::DoNothing());
   }