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).

https://github.com/krb5/krb5/commit/13fd26e1863c79f616653f6a10a58c01f65fceff
Author: Greg Hudson <ghudson@mit.edu>
Commit: 13fd26e1863c79f616653f6a10a58c01f65fceff
Branch: master
 src/lib/gssapi/spnego/spnego_mech.c |   15 +++++++++------
 1 files changed, 9 insertions(+), 6 deletions(-)