fix segfault if http {} block is empty, the bug had been introduced in r3218

src/http/ngx_http.c

diff --git a/src/http/ngx_http.c b/src/http/ngx_http.c
index 99ccef9..534fabf 100644
--- a/src/http/ngx_http.c
+++ b/src/http/ngx_http.c
@@ -1352,6 +1352,10 @@
     ngx_http_conf_port_t  *port;
     ngx_http_conf_addr_t  *addr;
 
+    if (ports == NULL) {
+        return NGX_OK;
+    }
+
     port = ports->elts;
     for (p = 0; p < ports->nelts; p++) {