krb5-gssapi: Removed a memory leak in krb5_auth.

We forgot to release the buffer passed to gss_init_sec_context.

The previous logic was difficult to read as we were reusing the same
variable (gssbuf) for both input buffer and output buffer. Splitted the
logic in 2 variables to better underline who needs to be released.
Also made the code break at 80 lines.