Fix QQmlExpression leaking QQmlError objects

If the user doesn't clear any potential errors manually via clearError(),
then do it automatically in the destructor. Found with valgrind.

[ChangeLog][QtQml][QQmlExpression] Fixed memory leak

Change-Id: If5b1181850c7463c939a7ba536d74e7054c53d60
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

src/qml/qml/qqmlexpression.cpp

@@ -202,6 +202,7 @@ QQmlExpression::QQmlExpression(QQmlContextData *ctxt, QObject *scope,
 */
 QQmlExpression::~QQmlExpression()
 {
+    clearError();
 }
 
 /*!