BUG/MEDIUM: resolvers: Remove aborted resolutions from query_ids tree

To avoid any UAF when a resolution is released, a mechanism was added to
abort a resolution and delayed the released at the end of the current
execution path. This mechanism depends on an hard assumption: Any reference
on an aborted resolution must be removed. So, when a resolution is aborted,
it is removed from the resolver lists and inserted into a death row list.

However, a resolution may still be referenced in the query_ids tree. It is
the tree containing all resolutions with a pending request. Because aborted
resolutions are released outside the resolvers lock, it is possible to
release a resolution on a side while a query ansswer is received and
processed on another one. Thus, it is still possible to have a UAF because
of this bug.

To fix the issue, when a resolution is aborted, it is removed from any list,
but it is also removed from the query_ids tree.

This patch should solve the issue #1862 and may be related to #1875. It must
be backported as far as 2.2.

src/resolvers.c

@@ -594,6 +594,11 @@ static void enter_resolver_code()
 /* Add a resolution to the death_row. */
 static void abort_resolution(struct resolv_resolution *res)
 {
+	/* Remove the resolution from query_ids tree and from any resolvers list */
+	eb32_delete(&res->qid);
+	res->query_id = 0;
+	res->qid.key   = 0;
+
 	LIST_DEL_INIT(&res->list);
 	LIST_APPEND(&death_row, &res->list);
 }