Fix ASN1_STRING_to_UTF8 could not convert NumericString

tag2nbyte had -1 at 18th position, but underlying ASN1_mbstring_copy
supports NumericString. tag2nbyte is also used in do_print_ex which will
not be broken by setting 1 at 18th position of tag2nbyte

Reviewed-by: Stephen Henson <steve@openssl.org>