Fix an NPE

A regression of ag/13066961.

'session' might be null inside the if block.
Use rollback.getStagedSessionId() to get the session ID instead.

Bug: 181087240
Test: manual test
Change-Id: Ie8bc2b451e5a1e5fd6fececccda7a3622c33a235

services/core/java/com/android/server/rollback/RollbackManagerServiceImpl.java

@@ -615,7 +615,7 @@ class RollbackManagerServiceImpl extends IRollbackManager.Stub implements Rollba
                 if (session == null || session.isStagedSessionFailed()) {
                     iter.remove();
                     deleteRollback(rollback,
-                            "Session " + session.getSessionId() + " not existed or failed");
+                            "Session " + rollback.getStagedSessionId() + " not existed or failed");
                     continue;
                 }