fellow_cache: support freeing an fco embedded in the fdo

src/fellow_cache.c

@@ -1640,13 +1640,13 @@ fellow_cache_obj_free(const struct fellow_cache *fc,
 	AZ(pthread_mutex_destroy(&fco->mtx));
 	AZ(pthread_cond_destroy(&fco->cond));
 
-	fellow_cache_seg_free(memret, fcs, 0);
-
 	if (fco->fco_dowry.bits)
 		AN(buddy_return_ptr_page(memret, &fco->fco_dowry));
 	TAKE(mem, fco->fco_mem);
-	assert(mem.ptr == fco);
-	AN(buddy_return_ptr_extent(memret, &mem));
+	// this frees the fdo, which may contain the fco
+	fellow_cache_seg_free(memret, fcs, 0);
+	if (mem.ptr)
+		AN(buddy_return_ptr_extent(memret, &mem));
 	buddy_return(memret);
 }