thread_win32.c: suppress warning

* thread_win32.c (native_sleep): constify local variable to suppress a
  false positive might-be-clobbered warning.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39523 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

thread_win32.c

@@ -301,14 +301,8 @@ rb_w32_Sleep(unsigned long msec)
 static void
 native_sleep(rb_thread_t *th, struct timeval *tv)
 {
-    DWORD msec;
-
-    if (tv) {
-	msec = tv->tv_sec * 1000 + tv->tv_usec / 1000;
-    }
-    else {
-	msec = INFINITE;
-    }
+    const DWORD msec = (tv) ?
+	(DWORD)(tv->tv_sec * 1000 + tv->tv_usec / 1000) : INFINITE;
 
     GVL_UNLOCK_BEGIN();
     {