Handle empty initial buffer in IAKERB initiator

Handle empty initial buffer in IAKERB initiator

Section 5.19 of RFC 2744 (about gss_init_sec_context) states,
"Initially, the input_token parameter should be specified either as
GSS_C_NO_BUFFER, or as a pointer to a gss_buffer_desc object whose
length field contains the value zero."  In iakerb_initiator_step(),
handle both cases when deciding whether to parse an acceptor message.

[ghudson@mit.edu: edited commit message]

ticket: 9126 (new)