Resolver: fixed use-after-free memory access.

In 954867a2f0a6, we switched to using resolver node as the
timer event data, so make sure we do not free resolver node
memory until the corresponding timer is deleted.

src/core/ngx_resolver.c

diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
index a17793b..7aa20ea 100644
--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -1568,8 +1568,6 @@
 
         ngx_rbtree_delete(&r->name_rbtree, &rn->node);
 
-        ngx_resolver_free_node(r, rn);
-
         /* unlock name mutex */
 
         while (next) {
@@ -1580,6 +1578,8 @@
             ctx->handler(ctx);
         }
 
+        ngx_resolver_free_node(r, rn);
+
         return;
     }
 
@@ -2143,8 +2143,6 @@
 
         ngx_rbtree_delete(tree, &rn->node);
 
-        ngx_resolver_free_node(r, rn);
-
         /* unlock addr mutex */
 
         while (next) {
@@ -2155,6 +2153,8 @@
             ctx->handler(ctx);
         }
 
+        ngx_resolver_free_node(r, rn);
+
         return;
     }