gssapi: add null checks for session->gssapi before using it

These are not strictly necessary because we always check
that we performed GSSAPI KEX, but they won't hurt us.

Signed-off-by: Pavol Žáčik <pzacik@redhat.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/auth.c

@@ -2501,6 +2501,11 @@ int ssh_userauth_gssapi_keyex(ssh_session session)
         return SSH_ERROR;
     }
 
+    if (session->gssapi == NULL) {
+        ssh_set_error(session, SSH_FATAL, "GSSAPI context not initialized");
+        return SSH_ERROR;
+    }
+
     rc = ssh_userauth_request_service(session);
     if (rc == SSH_AGAIN) {
         return SSH_AUTH_AGAIN;