fakeservicemanager: Don't accept null binders for addService

Parity with the rest of the Service Manager APIs.

Test: atest fakeservicemanager_test
Bug: 259549842
Change-Id: I715590bc4c4f2f434ad93f82c2ba98d40334ef38
diff --git a/libs/fakeservicemanager/ServiceManager.cpp b/libs/fakeservicemanager/ServiceManager.cpp
index 480ec79..deba724 100644
--- a/libs/fakeservicemanager/ServiceManager.cpp
+++ b/libs/fakeservicemanager/ServiceManager.cpp
@@ -36,6 +36,9 @@
 status_t ServiceManager::addService(const String16& name, const sp<IBinder>& service,
                                 bool /*allowIsolated*/,
                                 int /*dumpsysFlags*/) {
+    if (service == nullptr) {
+        return UNEXPECTED_NULL;
+    }
     mNameToService[name] = service;
     return NO_ERROR;
 }