Resolver: fixed possible use-after-free while resolving PTR.

Previously, if a response to the PTR request was cached, and ngx_resolver_dup()
failed to allocate memory for the resulting name, then the original node was
freed but left in expire_queue.  A subsequent address resolving would end up
in a use-after-free memory access of the node either in ngx_resolver_expire()
or ngx_resolver_process_ptr(), when accessing it through expire_queue.

The fix is to leave the resolver node intact.