fc: Clean up current config on shutdown

The FontConfig database had a static leak because we never
dereferenced the current config. To make address sanitizer output
less noisy, we clean this up on shutdown.

From FcConfigDestroy docs: "Note that calling this function with the
return from FcConfigGetCurrent will cause a new configuration to be
created for use as current configuration."

So this should be safe even if application execution continues after
Qt shutdown, but it could trigger creation of a new current config
in that case, if FontConfig calls are made.

Fixes: QTBUG-92477
Pick-to: 5.15 6.1 6.2
Change-Id: I596055a84edc1a1b06157e2adf6c8627c6802db1
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/gui/text/unix/qfontconfigdatabase.cpp

@@ -557,6 +557,11 @@ static void populateFromPattern(FcPattern *pattern, QFontDatabasePrivate::Applic
 
 }
 
+QFontconfigDatabase::~QFontconfigDatabase()
+{
+    FcConfigDestroy(FcConfigGetCurrent());
+}
+
 void QFontconfigDatabase::populateFontDatabase()
 {
     FcInit();

src/gui/text/unix/qfontconfigdatabase_p.h

@@ -61,6 +61,7 @@ class QFontEngineFT;
 class Q_GUI_EXPORT QFontconfigDatabase : public QFreeTypeFontDatabase
 {
 public:
+    ~QFontconfigDatabase() override;
     void populateFontDatabase() override;
     void invalidate() override;
     QFontEngineMulti *fontEngineMulti(QFontEngine *fontEngine, QChar::Script script) override;