Fix CVE-2014-3508, pretty printing and OID validation:
 - make sure the output buffer is always NUL terminated if buf_len
   was initially greater than zero.
 - reject OIDs that are too long, too short, or not in proper base-127

Based on
https://git.openssl.org/gitweb/?p=openssl.git;a=commit;h=0042fb5fd1c9d257d713b15a1f45da05cf5c1c87

ok bcook@