AK: Fix crash during teardown of self-owning objects

We now null out smart pointers *before* calling unref on the pointee.
This ensures that the same smart pointer can't be used to acquire a new
reference to the pointee after its destruction has begun.

I ran into this when destroying a non-empty IntrusiveList of RefPtrs,
but the problem was more general so this fixes it for all of RefPtr,
NonnullRefPtr, OwnPtr and NonnullOwnPtr.

Tests/AK/TestIntrusiveList.cpp

@@ -155,3 +155,9 @@ TEST_CASE(intrusive_nonnull_ref_ptr_intrusive)
 
     EXPECT(nonnull_ref_list.is_empty());
 }
+
+TEST_CASE(destroy_nonempty_intrusive_list)
+{
+    IntrusiveNonnullRefPtrList nonnull_ref_list;
+    nonnull_ref_list.append(adopt_ref(*new IntrusiveNonnullRefPtrItem));
+}