NAR parser: Fix missing name field check

Discovered by @Kloenk.

src/libutil/archive.cc

@@ -262,7 +262,7 @@ static void parse(ParseSink & sink, Source & source, const Path & path)
                             names[name] = 0;
                     }
                 } else if (s == "node") {
-                    if (s.empty()) throw badArchive("entry name missing");
+                    if (name.empty()) throw badArchive("entry name missing");
                     parse(sink, source, path + "/" + name);
                 } else
                     throw badArchive("unknown field " + s);