Verify decoded kadmin C strings [CVE-2015-8629]

In xdr_nullstring(), check that the decoded string is terminated with
a zero byte and does not contain any internal zero bytes.

CVE-2015-8629:

In all versions of MIT krb5, an authenticated attacker can cause
kadmind to read beyond the end of allocated memory by sending a string
without a terminating zero byte.  Information leakage may be possible
for an attacker with permission to modify the database.

    CVSSv2 Vector: AV:N/AC:H/Au:S/C:P/I:N/A:N/E:POC/RL:OF/RC:C

ticket: 8341 (new)
target_version: 1.14-next
target_version: 1.13-next
tags: pullup