QLocale: fix crash when using qDebug() in a global destructor

`kdevelop -s doesnotexist` led to
ASSERT failure in Q_GLOBAL_STATIC: "The global static was used after being destroyed
because of a qDebug() statement in some "unregister" method called by a
global object's destructor. This is normally fine, but with
%{time} in QT_MESSAGE_PATTERN, qDebug() ends up using QLocale after its
global objects (systemLocalePrivate and defaultLocalePrivate) were
destroyed.

Change-Id: I8d8b34e0197ad1eda8283fcf36d2c250385bb1a3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qlocale.cpp

@@ -2367,6 +2367,8 @@ QLocale QLocale::system()
 {
     // this function is NOT thread-safe!
     QT_PREPEND_NAMESPACE(systemData)(); // trigger updating of the system data if necessary
+    if (systemLocalePrivate.isDestroyed())
+        return QLocale(QLocale::C);
     return QLocale(*systemLocalePrivate->data());
 }