memory leak when use QTextDocument in the thread

valgrind detects memory leak for the attached example.
This doesn't happen when QTextDocument is used in the main thread.

Here is the detail:
==7558== 348,496 (720 direct, 347,776 indirect) bytes in 6 blocks are definitely lost in loss record 104 of 116
==7558== at 0x40277EE: operator new(unsigned) (in /usr/lib/valgrind/x86-linux/vgpreload_memcheck.so)
==7558== by 0x43FB1E5: QFontDatabase::load(QFontPrivate const*, int) (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x43D156C: QFontPrivate::engineForScript(int) const (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x4410385: QTextEngine::fontEngine(QScriptItem const&, QFixed*, QFixed*, QFixed*) const (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x44134B3: QTextEngine::shapeTextWithHarfbuzz(int) const (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x4414543: QTextEngine::shapeText(int) const (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x44149BB: QTextEngine::shape(int) const (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x441D249: QTextLine::layout_helper(int) (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x441E018: QTextLine::setLineWidth(double) (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x445D8E4: QTextDocumentLayoutPrivate::layoutBlock(QTextBlock const&, int, QTextBlockFormat const&, QLayoutStruct*, int, int, QTextBlockFormat const*) (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x44639F6: QTextDocumentLayoutPrivate::layoutFlow(QTextFrame::iterator, QLayoutStruct*, int, int, QFixed) (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x44621FD: QTextDocumentLayoutPrivate::layoutFrame(QTextFrame*, int, int, QFixed, QFixed, QFixed) (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)