BShelf: Unset the shelf of the containing view on destruction.

The BShelf sets itself as a handler on the containing view on creation
but didn't unset itself on destruction. When the containing view was
later destroyed it would unset the shelf which triggered a
use-after-free if you deleted the BShelf beforehand.

Since the ownership of a BShelf isn't explicitly documented I took the
shelf in DeskWindow of Tracker as a reference, which does delete it
explicitly.

src/kits/interface/Shelf.cpp

@@ -560,6 +560,8 @@ BShelf::~BShelf()
 		fReplicants.RemoveItem((int32)0);
 		delete data;
 	}
+
+	fContainerView->_SetShelf(NULL);
 }