Fix crash in storage when essential folders are missing from SDCARD

D/StorageMeasurement( 3622): getDirectorySize(/storage/primary/legacy) returned 3478597632

D/AndroidRuntime( 3622): Shutting down VM

W/dalvikvm( 3622): threadid=1: thread exiting with uncaught exception (group=0x40e71a08)

E/AndroidRuntime( 3622): FATAL EXCEPTION: main

E/AndroidRuntime( 3622): java.lang.NullPointerException

E/AndroidRuntime( 3622): 	at com.android.settings.deviceinfo.StorageVolumePreferenceCategory.totalValues(StorageVolumePreferenceCategory.java:313)

E/AndroidRuntime( 3622): 	at com.android.settings.deviceinfo.StorageVolumePreferenceCategory.updateDetails(StorageVolumePreferenceCategory.java:330)

E/AndroidRuntime( 3622): 	at com.android.settings.deviceinfo.StorageVolumePreferenceCategory$1.handleMessage(StorageVolumePreferenceCategory.java:96)