Add code-point range check to nqp:chr on jvm & throw error. Bring into line with pvm

To avoid the construction of invalid code-points that then later explode, e.g. on rakudo/jvm.

    my $c = chr(0xD800); # invalid code point. Should die here (imho)
    say("la la la");
    say($c);  #  currently exploding here.

I specced this from http://en.wikipedia.org/wiki/Mapping_of_Unicode_characters#Special_code_points.