src: fix -Wunused-result warning

PR-URL: #8450
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/node_util.cc

@@ -132,7 +132,7 @@ void Initialize(Local<Object> target,
 #define V(name, _)                                                            \
   target->Set(context,                                                        \
               FIXED_ONE_BYTE_STRING(env->isolate(), #name),                   \
-              Integer::NewFromUnsigned(env->isolate(), index++));
+              Integer::NewFromUnsigned(env->isolate(), index++)).FromJust();
   {
     uint32_t index = 0;
     PER_ISOLATE_PRIVATE_SYMBOL_PROPERTIES(V)