io.c (io_fflush): eliminate redundant rb_io_check_closed

There is no need to call this function twice in a row since
thread switching won't happen in-between calls to it.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66244 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

io.c

@@ -1158,7 +1158,6 @@ io_fflush(rb_io_t *fptr)
     rb_io_check_closed(fptr);
     if (fptr->wbuf.len == 0)
         return 0;
-    rb_io_check_closed(fptr);
     while (fptr->wbuf.len > 0 && io_flush_buffer(fptr) != 0) {
 	if (!rb_io_wait_writable(fptr->fd))
 	    return -1;