hw/misc/xlnx-versal-trng: Check returned bool in trng_prop_fault_event_set()

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 trng_prop_fault_event_set, @errp is dereferenced without
ERRP_GUARD():

visit_type_uint32(v, name, events, errp);
if (*errp) {
    return;
}

Currently, since trng_prop_fault_event_set() doesn't get the NULL @errp
parameter as a "set" method of object property, it hasn't triggered the
bug that dereferencing the NULL @errp.

And since visit_type_uint32() returns bool, check the returned bool
directly instead of dereferencing @errp, then we needn't the add missing
ERRP_GUARD().

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Message-Id: <20240223085653.1255438-5-zhao1.liu@linux.intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

hw/misc/xlnx-versal-trng.c

@@ -644,8 +644,7 @@ static void trng_prop_fault_event_set(Object *obj, Visitor *v,
     Property *prop = opaque;
     uint32_t *events = object_field_prop_ptr(obj, prop);
 
-    visit_type_uint32(v, name, events, errp);
-    if (*errp) {
+    if (!visit_type_uint32(v, name, events, errp)) {
         return;
     }