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

(cherry picked from commit df17a1224a3406f57477bcd372c61e04c0e5a5bb)

https://github.com/krb5/krb5/commit/54a9a01c6a923d604cd06f321b8381ebc5cb42d8
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 54a9a01c6a923d604cd06f321b8381ebc5cb42d8
Branch: krb5-1.14
 src/lib/kadm5/kadm_rpc_xdr.c |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)