display: Add a check when registering a client

This change checks if the registered client has a valid callback and
hwc session.

Change-Id: I9758d6d59c7087e67e89318746cb48054255ab72
CRs-Fixed: 2749805

services/config/src/device_impl.cpp

@@ -75,7 +75,13 @@ Return<void> DeviceImpl::registerClient(const hidl_string &client_name,
     callback->linkToDeath(this, client_handle);
   }
   ConfigInterface *intf = nullptr;
-  intf_->RegisterClientContext(device_client, &intf);
+  error = intf_->RegisterClientContext(device_client, &intf);
+
+  if (error) {
+    callback->unlinkToDeath(this);
+    return Void();
+  }
+
   device_client->SetDeviceConfigIntf(intf);
 
   display_config_map_.emplace(std::make_pair(client_handle, device_client));