QWinJumpListCategory: fix initialization of member variables

The pDocList member was left uninitialized when not using
QWinJumpListCategoryPrivate::create().

Change-Id: I3c443eaf00b8680d76cad710b2bdb8febb451ec7
Reviewed-by: Ivan Vizir <define-true-false@yandex.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>