hw/mem/cxl_type3: Fix missing ERRP_GUARD() in ct3_realize()

As the comment in qapi/error, dereferencing @errp requires
ERRP_GUARD():

* = Why, when and how to use ERRP_GUARD() =
*
* Without ERRP_GUARD(), use of the @errp parameter is restricted:
* - It must not be dereferenced, because it may be null.
...
* ERRP_GUARD() lifts these restrictions.
*
* To use ERRP_GUARD(), add it right at the beginning of the function.
* @errp can then be used without worrying about the argument being
* NULL or &error_fatal.
*
* Using it when it's not needed is safe, but please avoid cluttering
* the source with useless code.

But in ct3_realize(), @errp is dereferenced without ERRP_GUARD():

cxl_doe_cdat_init(cxl_cstate, errp);
if (*errp) {
    goto err_free_special_ops;
}

Here we check *errp, because cxl_doe_cdat_init() returns void. And
ct3_realize() - as a PCIDeviceClass.realize() method - doesn't get the
NULL @errp parameter, it hasn't triggered the bug that dereferencing
the NULL @errp.

To follow the requirement of @errp, add missing ERRP_GUARD() in
ct3_realize().

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20240223085653.1255438-4-zhao1.liu@linux.intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

hw/mem/cxl_type3.c

@@ -645,6 +645,7 @@ static DOEProtocol doe_cdat_prot[] = {
 
 static void ct3_realize(PCIDevice *pci_dev, Error **errp)
 {
+    ERRP_GUARD();
     CXLType3Dev *ct3d = CXL_TYPE3(pci_dev);
     CXLComponentState *cxl_cstate = &ct3d->cxl_cstate;
     ComponentRegisters *regs = &cxl_cstate->crb;