Resolver: fixed possible premature stop of the resend timer.

Previously, ngx_resolve_name_done() and ngx_resolve_addr_done()
may have stopped the resend timer prematurely while srv_resend_queue
was not empty.

src/core/ngx_resolver.c

diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
index fc25c66..d5f06eb 100644
--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -1556,6 +1556,7 @@
 ngx_resolver_resend_empty(ngx_resolver_t *r)
 {
     return ngx_queue_empty(&r->name_resend_queue)
+           && ngx_queue_empty(&r->srv_resend_queue)
 #if (NGX_HAVE_INET6)
            && ngx_queue_empty(&r->addr6_resend_queue)
 #endif