Resolver: fixed possible use-after-free in worker on fast shutdown.

src/core/ngx_resolver.c

diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
index bdfed88..834196a 100644
--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -300,6 +300,10 @@
 #endif
 
         if (r->event) {
+            if (r->event->timer_set) {
+                ngx_del_timer(r->event);
+            }
+
             ngx_free(r->event);
         }