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

The fix in a3dc657f4e95 was incomplete.

src/core/ngx_resolver.c

diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
index 834196a..fc25c66 100644
--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -351,6 +351,10 @@
             next = ctx->next;
 
             if (ctx->event) {
+                if (ctx->event->timer_set) {
+                    ngx_del_timer(ctx->event);
+                }
+
                 ngx_resolver_free(r, ctx->event);
             }