format_to_n unsafe when arguments exceeds some size

[arthurfait]

format_to_n unsafe when arguments exceeds some size (256?)

Relates to closed #2028 (#1996)

Following peace of code

#include <fmt/format.h>
int main()
{
    char buff[80];
    const char *a = "11111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111";
    fmt::format_to_n(std::begin(buff), 79,FMT_STRING("{} "), a);
    fmt::print("done");
    return 0;
}

build with following rule:
-fstack-protector-strong

gives

*** stack smashing detected ***: terminated

[vitaut]

Great catch, thanks! Fixed in 2c734c9.