Fixed typo introduced in 81040de6b085.

Found by Coverity (CID 1472504, CID 1472505).
diff --git a/nginx/ngx_js_fetch.c b/nginx/ngx_js_fetch.c
index bcaf856..8d9d16f 100644
--- a/nginx/ngx_js_fetch.c
+++ b/nginx/ngx_js_fetch.c
@@ -467,7 +467,7 @@
             start++;
 
             value = njs_vm_object_prop(vm, headers, &name, &lvalue);
-            if (ret != NJS_OK) {
+            if (value == NULL) {
                 goto fail;
             }