Resolver: require name servers.

src/core/ngx_resolver.c

diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
index 1e4b411..d969704 100644
--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -275,6 +275,11 @@
         }
     }
 
+    if (n && r->connections.nelts == 0) {
+        ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "no name servers defined");
+        return NULL;
+    }
+
     return r;
 }