CMFileManager: protect code path for access mSdBookmarks

FATAL EXCEPTION: main

AndroidRuntime: Process: com.cyanogenmod.filemanager, PID: 2587

AndroidRuntime: java.lang.NullPointerException: Attempt to invoke interface method 'java.util.Iterator java.util.List.iterator()' on a null object reference

AndroidRuntime: 	at com.cyanogenmod.filemanager.activities.NavigationActivity.applyInitialDir(NavigationActivity.java:1626)

AndroidRuntime: 	at com.cyanogenmod.filemanager.activities.NavigationActivity$15.run(NavigationActivity.java:1521)

AndroidRuntime: 	at android.os.Handler.handleCallback(Handler.java:739)

Change-Id: I2aec4fd6a5b8fcd31cd128f8f46cc9f88bca191e
(cherry picked from commit 05ddc9cd)