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: c5475b3 ("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>
(cherry picked from commit c8f48b1)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/misc/aspeed_hace.c

@@ -189,7 +189,7 @@ static void do_hash_operation(AspeedHACEState *s, int algo, bool sg_mode,
                               bool acc_mode)
 {
     struct iovec iov[ASPEED_HACE_MAX_SG];
-    g_autofree uint8_t *digest_buf;
+    g_autofree uint8_t *digest_buf = NULL;
     size_t digest_len = 0;
     int niov = 0;
     int i;