core/qdev: fix memleak in qdev_get_gpio_out_connector()

Fix a memory leak in qdev_get_gpio_out_connector().

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Pan Nengyuan <pannengyuan@huawei.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200307030756.5913-1-pannengyuan@huawei.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>