Work around bogus clang -Wint-conversion warning.

Index: src/util/virthread.h
--- src/util/virthread.h.orig
+++ src/util/virthread.h
@@ -212,7 +212,7 @@ int virThreadLocalSet(virThreadLocal *l, void*) G_GNUC
 
 #define VIR_WITH_MUTEX_LOCK_GUARD_(m, name) \
     for (g_auto(virLockGuard) name = virLockGuardLock(m); name.mutex; \
-        name.mutex = (virLockGuardUnlock(&name), NULL))
+        virLockGuardUnlock(&name), name.mutex = NULL)
 /**
  * VIR_WITH_MUTEX_LOCK_GUARD:
  *