Fix possible crash in KDirModel if the root item is deleted or moved

If the root item of the dir model is deleted, but it is not the first
item in the list that KDirModelPrivate::_k_slotDeleteItems(...) gets
from the dir lister, a crash may result because
KDirModelPrivate::removeFromNodeHash(...) calls isDir() for a null
KFileItem. This commit extends the protection agains this kind of crash
that has been introduced in c8939409eed00420fb43ff22cfc6c9092e4da7e5 for
the first item to the rest of the list.