Set timeout to nil if unspecified

Author: nobu

ext/io/wait/wait.c

@@ -243,6 +243,7 @@ io_wait(int argc, VALUE *argv, VALUE io)
 rb_raise(rb_eArgError, "timeout given more than once");
  }
 }
+if (timeout == Qundef) timeout = Qnil;
  }
  else /* argc == 2 */ {
 events = RB_NUM2UINT(argv[0]);