Do not register with null observer

bug: 168307955
test: unit tests.
Change-Id: Iadef8660c95f2c545a212dfabf06c264b7c719a6
diff --git a/services/mediaresourcemanager/ResourceObserverService.cpp b/services/mediaresourcemanager/ResourceObserverService.cpp
index 9cc6fe4..4e97406 100644
--- a/services/mediaresourcemanager/ResourceObserverService.cpp
+++ b/services/mediaresourcemanager/ResourceObserverService.cpp
@@ -165,6 +165,10 @@
         return Status::fromServiceSpecificError(PERMISSION_DENIED);
     }
 
+    if (in_observer == nullptr) {
+        return Status::fromServiceSpecificError(BAD_VALUE);
+    }
+
     ::ndk::SpAIBinder binder = in_observer->asBinder();
 
     {
@@ -220,6 +224,10 @@
         return Status::fromServiceSpecificError(PERMISSION_DENIED);
     }
 
+    if (in_observer == nullptr) {
+        return Status::fromServiceSpecificError(BAD_VALUE);
+    }
+
     ::ndk::SpAIBinder binder = in_observer->asBinder();
 
     {