src: combine loops in CopyJsStringArray()

In spawn_sync.cc, two consecutive loops are used to convert
data to strings, and compute the size of the data. This commit
merges the two independent loops into one.

Backport-PR-URL: #16426
Refs: #15380
PR-URL: #16247
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/spawn_sync.cc

@@ -1016,25 +1016,22 @@ int SyncProcessRunner::CopyJsStringArray(Local<Value> js_value,
   Local<Context> context = env()->context();
   js_array = js_value.As<Array>()->Clone().As<Array>();
   length = js_array->Length();
+  data_size = 0;
+
+  // Index has a pointer to every string element, plus one more for a final
+  // null pointer.
+  list_size = (length + 1) * sizeof *list;
 
   // Convert all array elements to string. Modify the js object itself if
-  // needed - it's okay since we cloned the original object.
+  // needed - it's okay since we cloned the original object. Also compute the
+  // length of all strings, including room for a null terminator after every
+  // string. Align strings to cache lines.
   for (uint32_t i = 0; i < length; i++) {
     auto value = js_array->Get(context, i).ToLocalChecked();
 
     if (!value->IsString())
       js_array->Set(context, i, value->ToString(env()->isolate())).FromJust();
-  }
 
-  // Index has a pointer to every string element, plus one more for a final
-  // null pointer.
-  list_size = (length + 1) * sizeof *list;
-
-  // Compute the length of all strings. Include room for null terminator
-  // after every string. Align strings to cache lines.
-  data_size = 0;
-  for (uint32_t i = 0; i < length; i++) {
-    auto value = js_array->Get(context, i).ToLocalChecked();
     data_size += StringBytes::StorageSize(isolate, value, UTF8) + 1;
     data_size = ROUND_UP(data_size, sizeof(void*));
   }