Prevent EBCDIC overread for very long strings

ASN1 Strings that are over 1024 bytes can cause an overread in
applications using the X509_NAME_oneline() function on EBCDIC systems.
This could result in arbitrary stack data being returned in the buffer.

Issue reported by Guido Vranken.

CVE-2016-2176

Reviewed-by: Andy Polyakov <appro@openssl.org>