binderRpcTest: actually include strong count in leaked ref

Bug: 244325464
Change-Id: I65da2532fd2807849aaba2ea1c41964e736d791d
Test: binderRpcTest
diff --git a/libs/binder/tests/binderRpcTest.cpp b/libs/binder/tests/binderRpcTest.cpp
index 7a550f7..652da99 100644
--- a/libs/binder/tests/binderRpcTest.cpp
+++ b/libs/binder/tests/binderRpcTest.cpp
@@ -181,9 +181,10 @@
             wp<RpcSession> weakSession = session;
             session = nullptr;
 
-            EXPECT_EQ(nullptr, weakSession.promote())
+            sp<RpcSession> strongSession = weakSession.promote();
+            EXPECT_EQ(nullptr, strongSession)
                     << (debugBacktrace(host.getPid()), debugBacktrace(getpid()), "Leaked sess: ")
-                    << session->getStrongCount();
+                    << strongSession->getStrongCount();
         }
     }
 };