Fixed potential null pointer dereference in ngx_resolver_create().
While here, improved error message.

src/core/ngx_resolver.c

diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
index edc43dc..54bd384 100644
--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -175,7 +175,12 @@
         u.port = 53;
 
         if (ngx_inet_resolve_host(cf->pool, &u) != NGX_OK) {
-            ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "%V: %s", &u.host, u.err);
+            if (u.err) {
+                ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
+                                   "%s in resolver \"%V\"",
+                                   u.err, &u.host);
+            }
+
             return NULL;
         }