util: Add object checking for virObject{Ref|Unref}

Rather than assuming that what's passed to virObject{Ref|Unref}
would be a virObjectPtr as long as it's not NULL, let's do the
similar checks virObjectIsClass in order to prevent a possible
increment or decrement to some field at the obj->u.s.refs offset.

Signed-off-by: John Ferlan <jferlan@redhat.com>

src/util/virobject.c

@@ -343,7 +343,7 @@ virObjectUnref(void *anyobj)
 {
     virObjectPtr obj = anyobj;
 
-    if (!obj)
+    if (VIR_OBJECT_NOTVALID(obj))
         return false;
 
     bool lastRef = virAtomicIntDecAndTest(&obj->u.s.refs);
@@ -382,7 +382,7 @@ virObjectRef(void *anyobj)
 {
     virObjectPtr obj = anyobj;
 
-    if (!obj)
+    if (VIR_OBJECT_NOTVALID(obj))
         return NULL;
     virAtomicIntInc(&obj->u.s.refs);
     PROBE(OBJECT_REF, "obj=%p", obj);