QSystemLocale: bail out if accessed post-destruction

There's little we can do, but a lot of content ends up in QLocale very
late in the execution. Let's at least not crash.

Task-number: QTBUG-133206
Pick-to: 6.9 6.8
Change-Id: I77d41141cb115147f9befffdd5e69dac19c96044
Reviewed-by: Albert Astals Cid <aacid@kde.org>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/text/qlocale_unix.cpp

@@ -127,6 +127,8 @@ QLocale QSystemLocale::fallbackLocale() const
 QVariant QSystemLocale::query(QueryType type, QVariant &&in) const
 {
     QSystemLocaleData *d = qSystemLocaleData();
+    if (!d)
+        return QVariant();
 
     if (type == LocaleChanged) {
         d->readEnvironment();

src/corelib/text/qlocale_win.cpp

@@ -870,6 +870,8 @@ QLocale QSystemLocale::fallbackLocale() const
 QVariant QSystemLocale::query(QueryType type, QVariant &&in) const
 {
     QSystemLocalePrivate *d = systemLocalePrivate();
+    if (!d)
+        return QVariant();
     switch(type) {
     case DecimalPoint:
         return d->decimalPoint();