Merge "Assert if alarm callback is null"

  if (alarm == NULL) {

    return;  // The alarm was probably canceled

  }

  //

  // If the alarm is not periodic, we've fully serviced it now, and can reset

  // some of its internal state. This is useful to distinguish between expired

  // alarms and active ones.

  //

  if (!alarm->callback) {

    LOG(FATAL) << __func__

               << ": timer callback is NULL! Name=" << alarm->stats.name;

  }

  alarm_callback_t callback = alarm->callback;

  void* data = alarm->data;

  uint64_t deadline_ms = alarm->deadline_ms;