Fix an NPE when cancelling full backup in BMS#endFullBackup()

Bug: 63000171
Test: it's just a nullcheck that compiles
Change-Id: I90faa5577f2fd656bb4f7b6af39494590a77e3bf
diff --git a/services/backup/java/com/android/server/backup/BackupManagerService.java b/services/backup/java/com/android/server/backup/BackupManagerService.java
index 30de4ba..6f6e1b7 100644
--- a/services/backup/java/com/android/server/backup/BackupManagerService.java
+++ b/services/backup/java/com/android/server/backup/BackupManagerService.java
@@ -5666,13 +5666,15 @@
                 PerformFullTransportBackupTask pftbt = null;
                 synchronized (mQueueLock) {
                     if (mRunningFullBackupTask != null) {
-                        if (DEBUG_SCHEDULING) {
-                            Slog.i(TAG, "Telling running backup to stop");
-                        }
                         pftbt = mRunningFullBackupTask;
                     }
                 }
-                pftbt.handleCancel(true);
+                if (pftbt != null) {
+                    if (DEBUG_SCHEDULING) {
+                        Slog.i(TAG, "Telling running backup to stop");
+                    }
+                    pftbt.handleCancel(true);
+                }
             }
         };
         new Thread(endFullBackupRunnable, "end-full-backup").start();