Fix potential data race

This case could be considered benign however it could still be an
undefined behavior to the compiler, so remove it.

Ref:
https://hacks.mozilla.org/2021/04/eliminating-data-races-in-firefox-a-technical-report/

src/base/settingsstorage.cpp

@@ -184,9 +184,8 @@ SettingsStorage *SettingsStorage::instance()
 
 bool SettingsStorage::save()
 {
-    if (!m_dirty) return true; // Obtaining the lock is expensive, let's check early
+    const QWriteLocker locker(&m_lock);  // guard for `m_dirty` too
-    const QWriteLocker locker(&m_lock);  // to guard for `m_dirty`
+    if (!m_dirty) return true;
-    if (!m_dirty) return true; // something might have changed while we were getting the lock
 
     const TransactionalSettings settings(QLatin1String("qBittorrent"));
     if (!settings.write(m_data))