Fix null deref in KDC when decoding invalid NDR

In ndr_dec_delegation_info(), keep di->transited_services_length valid
by incrementing it as we add entries.  Otherwise
ndr_free_delegation_info() could dereference a null
di->transited_services field.  Also bound nservices using data->length
to prevent inordinately large memory allocations.  Credit to OSS-Fuzz
for discovering the issues.

(cherry picked from commit fa62bd33a0c0889c083999c0289ffa81a5d51e7b)

https://github.com/krb5/krb5/commit/fdf193f188005c8ed4cb27558425052b673f89a1
Author: Greg Hudson <ghudson@mit.edu>
Commit: fdf193f188005c8ed4cb27558425052b673f89a1
Branch: krb5-1.20
 src/kdc/ndr.c   | 14 +++++++-------
 src/kdc/t_ndr.c | 32 +++++++++++++++++++++++++++++---
 2 files changed, 36 insertions(+), 10 deletions(-)