The failure paths in CDAT file loading did not clear up properly.
Change to using g_auto_free and a local pointer for the buffer to
ensure this function has no side effects on error.
Also drop some unnecessary checks that can not fail.

Cleanup properly after a failure to load a CDAT file.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20230421132020.7408-3-Jonathan.Cameron@huawei.com>
Reviewed-by: Fan Ni <fan.ni@samsung.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>