Avoid malloc(0) in SPNEGO get_input_token

If we read a zero-length token in spnego_mech.c's get_input_token(),
set the value pointer to NULL instead of calling malloc(0).

(cherry picked from commit 13fd26e1863c79f616653f6a10a58c01f65fceff)

https://github.com/krb5/krb5/commit/091a9614049ace17fd89352c2d7e42ff210991ed
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 091a9614049ace17fd89352c2d7e42ff210991ed
Branch: krb5-1.12
 src/lib/gssapi/spnego/spnego_mech.c |   15 +++++++++------
 1 files changed, 9 insertions(+), 6 deletions(-)