Fix parsing 0 parameters when using choices

[ebothmann]

Before, when an option used choices, but the user passed no argument, instead of giving the too-few-arguments given error, a bad any cast exception was thrown. This is now fixed by not accidentally changing num_args_max to 0 before the number-of-arguments check.

E.g. running ./program --color (i.e. no arguments)

    parser.add_argument("--color")
        .help("configure the use of colors in output")
        .choices("auto", "always", "never");

caused the (unhelpful) exception

libc++abi: terminating due to uncaught exception of type std::bad_any_cast: bad any cast

at the line

auto fn = parser.present("--color");

With the change, the same invocation gives us the much more helpful

Too few arguments for '--color'.