fix bug of fmt overflow

[Valentin V. Bartenev]

On Saturday, 10 November 2018 16:18:25 MSK wy7980 wrote:
> Yes, I just mean the incorrect format string of fmt, it can be dangerous.
[..]

If the format string is incorrect, e.g. if it isn't terminated with a null
character or have incorrect specifiers, or the number of them is bigger than
the number of supplied arguments then it's dangerous anyway (with or without
your patch).

So, please don't supply incorrect format string in your code.

  wbr, Valentin V. Bartenev