can partially read wtp descriptor

FossilOrigin-Name: 87c986decab2956c060dc9212e5bd4d89bed829ec528477817f8ab795c396853

src/cw/mavl_replace.c

@@ -3,7 +3,8 @@
 union mavldata *mavl_replace(struct mavl *t,union mavldata *data){
 	struct mavlnode * node = mavl_get_node(t,data);
 	if (node){
-		t->del(&node->data);
+		if (t->del) 
+			t->del(&node->data);
 		node->data=*data;
 		return &node->data;	
 	}