Check for NULL before calling asBinder().

The old asBinder() checked if this == NULL, which is undefined behavior.

Change-Id: Ibde628395ca10dfef0d2f59e81280576f104b83c
diff --git a/drm/common/IDrmManagerService.cpp b/drm/common/IDrmManagerService.cpp
index db41e0b..36cb612 100644
--- a/drm/common/IDrmManagerService.cpp
+++ b/drm/common/IDrmManagerService.cpp
@@ -148,7 +148,8 @@
 
     data.writeInterfaceToken(IDrmManagerService::getInterfaceDescriptor());
     data.writeInt32(uniqueId);
-    data.writeStrongBinder(drmServiceListener->asBinder());
+    data.writeStrongBinder(
+        drmServiceListener != NULL ? drmServiceListener->asBinder() : NULL);
     remote()->transact(SET_DRM_SERVICE_LISTENER, data, &reply);
     return reply.readInt32();
 }