aspeed/hace: Initialize g_autofree pointer

aspeed/hace: Initialize g_autofree pointer

As mentioned in docs/devel/style.rst "Automatic memory deallocation":

* Variables declared with g_auto* MUST always be initialized,
  otherwise the cleanup function will use uninitialized stack memory

This avoids QEMU to coredump when running the "hash test" command
under Zephyr.

Cc: Steven Lee <steven_lee@aspeedtech.com>
Cc: Joel Stanley <joel@jms.id.au>
Cc: qemu-stable@nongnu.org
Fixes: c5475b3f9a ("hw: Model ASPEED's Hash and Crypto Engine")
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Message-Id: <20230421131547.2177449-1-clg@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Cédric Le Goater <clg@kaod.org>