Fix segfault on IsOOBEComplete() Android Stub.

The Android stub implementation of IsOOBEComplete() didn't check if the
passed pointer is a nullptr. The parameter can be a nullptr when the
time the OOBE was completed is not interesting.

Bug: 22024447
Test: `mma`; deployed on dragonboard and ran `update_engine_client --update`.

Change-Id: Ie680e9d1327717ca6618d8154825767815e04196
diff --git a/hardware_android.cc b/hardware_android.cc
index 5c76989..73fef99 100644
--- a/hardware_android.cc
+++ b/hardware_android.cc
@@ -49,7 +49,8 @@
 
 bool HardwareAndroid::IsOOBEComplete(base::Time* out_time_of_oobe) const {
   LOG(WARNING) << "STUB: Assuming OOBE is complete.";
-  *out_time_of_oobe = base::Time();
+  if (out_time_of_oobe)
+    *out_time_of_oobe = base::Time();
   return true;
 }