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.

src/core/ngx_resolver.c

@@ -972,7 +972,8 @@ ngx_resolve_addr(ngx_resolver_ctx_t *ctx)
 
             name = ngx_resolver_dup(r, rn->name, rn->nlen);
             if (name == NULL) {
-                goto failed;
+                ngx_resolver_free(r, ctx);
+                return NGX_ERROR;
             }
 
             ctx->name.len = rn->nlen;