fix reverse resolving cache: it stored zero length names
*) free name only if it has been already allocated
*) store name length

src/core/ngx_resolver.c

diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
index 05b44c7..ce94201 100644
--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -1523,7 +1523,11 @@
 
     if (len != (size_t) rn->nlen || ngx_strncmp(name.data, rn->name, len) != 0)
     {
-        ngx_resolver_free(r, rn->name);
+        if (rn->nlen) {
+            ngx_resolver_free(r, rn->name);
+        }
+
+        rn->nlen = len;
         rn->name = name.data;
 
         name.data = ngx_resolver_dup(r, rn->name, len);