Work around bogus clang -Wint-conversion warning.

Index: src/util/virobject.h
--- src/util/virobject.h.orig
+++ src/util/virobject.h
@@ -145,7 +145,7 @@ virObjectListFreeCount(void *list,
 
 #define VIR_WITH_OBJECT_LOCK_GUARD_(o, name) \
     for (g_auto(virLockGuard) name = virObjectLockGuard(o); name.mutex; \
-         name.mutex = (virLockGuardUnlock(&name), NULL))
+         virLockGuardUnlock(&name), name.mutex = NULL)
 
 /**
  * VIR_WITH_OBJECT_LOCK_GUARD: