fix return code, this fixes segfault when two or more
simultaneous connections are resolving the same address

src/core/ngx_resolver.c

diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
index cbc6505..c68c7f4 100644
--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -626,7 +626,9 @@
             ctx->next = rn->waiting;
             rn->waiting = ctx;
 
-            return NGX_AGAIN;
+            /* unlock addr mutex */
+
+            return NGX_OK;
         }
 
         ngx_queue_remove(&rn->queue);