Don't crash on transport unregistration if there is no current transport

Change-Id: Icd0962254ba224a7be27d408dffb25b634f67863
diff --git a/services/java/com/android/server/BackupManagerService.java b/services/java/com/android/server/BackupManagerService.java
index ee68a50b5..62dcb08 100644
--- a/services/java/com/android/server/BackupManagerService.java
+++ b/services/java/com/android/server/BackupManagerService.java
@@ -641,7 +641,7 @@
                 mTransports.put(name, transport);
             } else {
                 mTransports.remove(name);
-                if (mCurrentTransport.equals(name)) {
+                if ((mCurrentTransport != null) && mCurrentTransport.equals(name)) {
                     mCurrentTransport = null;
                 }
                 // Nothing further to do in the unregistration case