hw/pci-bridge/cxl_upstream: Fix missing ERRP_GUARD() in cxl_usp_realize()

hw/pci-bridge/cxl_upstream: Fix missing ERRP_GUARD() in cxl_usp_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 cxl_usp_realize(), @errp is dereferenced without ERRP_GUARD():

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

Here we check *errp, because cxl_doe_cdat_init() returns void. And since
cxl_usp_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
cxl_usp_realize().

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20240223085653.1255438-6-zhao1.liu@linux.intel.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>