krb5: fix CVE-2015-2697

The build_principal_va function in lib/krb5/krb/bld_princ.c in MIT
Kerberos 5 (aka krb5) before 1.14 allows remote authenticated users
to cause a denial of service (out-of-bounds read and KDC crash) via
an initial '\0' character in a long realm field within a TGS request.

Backport upstream commit to fix it:
https://github.com/krb5/krb5/commit/f0c094a1b745d91ef2f9a4eae2149aac026a5789



Signed-off-by: Wenzong Fan <wenzong.fan@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>