comment FreeBSD PR number.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@9808 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

eval.c

@@ -9644,7 +9644,7 @@ rb_fd_isset(n, fds)
     const rb_fdset_t *fds;
 {
     if (n >= fds->maxfd) return 0;
-    return FD_ISSET(n, fds->fdset) != 0; /* "!= 0" avoids FreeBSD bug */
+    return FD_ISSET(n, fds->fdset) != 0; /* "!= 0" avoids FreeBSD PR 91421 */
 }
 
 void