Fix memory leak in SPNEGO gss_init_sec_context()

After the initial call to spnego_gss_init_sec_context(), the context
handle can leak if init_ctx_cont() returns an error, because the
cleanup handler assumes that spnego_ctx contains the value of
*context_handle.  Fix this leak by setting spnego_ctx before the if
block which contains that call.  Reported by Adam Bernstein.

(cherry picked from commit 159dbbd5ff14fdc2fa71fb3a8804eb401c914399)

https://github.com/krb5/krb5/commit/fd0eb62f85e1f57d07aec72bec73a90f5032b60b
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: fd0eb62f85e1f57d07aec72bec73a90f5032b60b
Branch: krb5-1.13
 src/lib/gssapi/spnego/spnego_mech.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)