Character set checking for few of ASN.1 strings.

It's unclear which characters are valid in VisibleString and IA5String (based on locale etc.), but their value should be under 128, so it's worth checking.
I ran it against testtube (1379 certificates) and with checking character sets all parsing takes 1.74757814407 
(so 0.00126727929 on average) vs 1.69604635239 (0.00122991033531 on average) without, so I guess it's not that bad.