health 2.0: Check health_loop status for passthrough usage

health HAL is used as passthrough in recovery mode and hence
healthd_main() is not invoked. Add checks in
healthd_battery_update_internal() and healthd_register_event()
to return when health_loop is NULL.

Test: fastboot getvar battery-voltage works in fastbootd.
Bug: 144299329

Change-Id: I0a975a4bfe942813f9c5b3936df91cc3d8fa9674
diff --git a/health/2.0/default/healthd_common_adapter.cpp b/health/2.0/default/healthd_common_adapter.cpp
index 8fc689d..0b5d869 100644
--- a/health/2.0/default/healthd_common_adapter.cpp
+++ b/health/2.0/default/healthd_common_adapter.cpp
@@ -49,11 +49,14 @@
 static std::unique_ptr<HealthLoopAdapter> health_loop;
 
 int healthd_register_event(int fd, void (*handler)(uint32_t), EventWakeup wakeup) {
+    if (!health_loop) return -1;
+
     auto wrapped_handler = [handler](auto*, uint32_t epevents) { handler(epevents); };
     return health_loop->RegisterEvent(fd, wrapped_handler, wakeup);
 }
 
 void healthd_battery_update_internal(bool charger_online) {
+    if (!health_loop) return;
     health_loop->AdjustWakealarmPeriods(charger_online);
 }