Fix NPE

NPE happens when there is an orphaned session which we've
tried to prevent in all cases.

Log an error message if this situation happens.

Bug: 227342978
Test: atest CtsRootPackageInstallerHostTestCases
Change-Id: Ia21323926bd9db1a6f05461904deb45b4c3dd0bc

services/core/java/com/android/server/pm/PackageInstallerService.java

@@ -410,7 +410,10 @@ public class PackageInstallerService extends IPackageInstaller.Stub implements
                     // Their staging dirs will be removed too
                     PackageInstallerSession root = !session.hasParentSessionId()
                             ? session : mSessions.get(session.getParentSessionId());
-                    if (!root.isDestroyed()) {
+                    if (root == null) {
+                        Slog.e(TAG, "freeStageDirs: found an orphaned session: "
+                                + session.sessionId + " parent=" + session.getParentSessionId());
+                    } else if (!root.isDestroyed()) {
                         root.abandon();
                     }
                 } else {