Fix NullPointerException.

Fix a NullPointerException that can occur if a package with a recently
created rollback is no longer installed on device when the device boots.

Bug: 137135842
Test: atest CtsRollbackManagerHostTestCases
Change-Id: Ic007e44264647793c6271c3923c13c9e1736a9de

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

@@ -1289,7 +1289,8 @@ class RollbackManagerServiceImpl extends IRollbackManager.Stub {
 
 
     private boolean packageVersionsEqual(VersionedPackage a, VersionedPackage b) {
-        return a.getPackageName().equals(b.getPackageName())
+        return a != null && b != null
+            && a.getPackageName().equals(b.getPackageName())
             && a.getLongVersionCode() == b.getLongVersionCode();
     }