Don't pass along malparsed private messages.

Was seeing some crashes resulting from nil-ID private messages escaping.
Obviously this should be fixed before this point.
Keep strong references to posts view controller's views.

This stops us from loading views in -dealloc and may prevent some iOS 5 crashes.
Image preview keeps strong references to views.

Same as earlier changes to posts view: might fix an iOS 5 crash in
objc_loadWeak, and should fix a crash where a scroll view's delegate gets
deallocated.