Do not retrieve zero-length creds

In the MSLSA cache, if we get back a zero-length ticket, don't
accept it as success; continue on to try and get an acceptable
ticket.

(cherry picked from commit cd58adf4b0f52d0293ec8bf9d7d3e87bd1e6ce3f)

https://github.com/krb5/krb5/commit/4e51366fc61b54ca84b3f48cd2abc0ba62852a7b
Author: Ben Kaduk <kaduk@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 4e51366fc61b54ca84b3f48cd2abc0ba62852a7b
Branch: krb5-1.10
 src/lib/krb5/ccache/cc_mslsa.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)