Fix for market-reported NPE:

java.lang.NullPointerException
at com.fsck.k9.controller.MessagingController.actOnMessages(MessagingController.java:5602)
at com.fsck.k9.controller.MessagingController.deleteThreads(MessagingController.java:3986)
at com.fsck.k9.fragment.MessageListFragment.onDelete(MessageListFragment.java:1311)
at com.fsck.k9.fragment.MessageListFragment.onDelete(MessageListFragment.java:1306)
at com.fsck.k9.fragment.MessageListFragment.onContextItemSelected(MessageListFragment.java:1506)
at android.support.v4.app.Fragment.performContextItemSelected(Fragment.java:1583)
at android.support.v4.app.FragmentManagerImpl.dispatchContextItemSelected(FragmentManager.java:1992)
at android.support.v4.app.FragmentActivity.onMenuItemSelected(FragmentActivity.java:370)
at com.actionbarsherlock.app.SherlockFragmentActivity.onMenuItemSelected(SherlockFragmentActivity.java:211)
at com.android.internal.policy.impl.PhoneWindow$DialogMenuCallback.onMenuItemSelected(PhoneWindow.java:4038)
at com.android.internal.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:735)
at com.android.internal.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:149)
at com.android.internal.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:874)
at com.android.internal.view.menu.MenuDialogHelper.onClick(MenuDialogHelper.java:193)
at com.android.internal.app.AlertController$AlertParams$3.onItemClick(AlertController.java:934)
at android.widget.AdapterView.performItemClick(AdapterView.java:301)
at android.widget.AbsListView.performItemClick(AbsListView.java:1287)
at android.widget.AbsListView$PerformClick.run(AbsListView.java:3078)
at android.widget.AbsListView$1.run(AbsListView.java:4161)
at android.os.Handler.handleCallback(Handler.java:615)
at android.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4921)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1038)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:805)
at dalvik.system.NativeStart.main(Native Method)