QTextFormat: fix undefined behavior

Left-shifting of negative values is undefined ([expr.shift]/2).

Since hashValue is a uint already, rectify by casting it->key to
uint prior to shifting.

Found by UBSan.

Change-Id: I94a5311f5a4492f514f595b8fb79726df1e7d0de
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/gui/text/qtextformat.cpp

@@ -319,7 +319,7 @@ uint QTextFormatPrivate::recalcHash() const
 {
     hashValue = 0;
     for (QVector<Property>::ConstIterator it = props.constBegin(); it != props.constEnd(); ++it)
-        hashValue += (it->key << 16) + variantHash(it->value);
+        hashValue += (static_cast<quint32>(it->key) << 16) + variantHash(it->value);
 
     hashDirty = false;