gss: make krb5 default GSS mech earlier in path

Defensively default to GSS_KRB5_MECHANISM on all calls to
gss_init_sec_context() if mech_type is GSS_C_NO_OID (rather than only on the
first call).

lib/gssapi/mech/gss_init_sec_context.c

@@ -173,6 +173,9 @@ gss_init_sec_context(OM_uint32 * minor_status,
 	if (time_rec)
 	    *time_rec = 0;
 
+	if (mech_type == GSS_C_NO_OID)
+	    mech_type = GSS_KRB5_MECHANISM;
+
 	_gss_mg_check_name(target_name);
 
 	if (_gss_mg_log_level(1))
@@ -186,9 +189,6 @@ gss_init_sec_context(OM_uint32 * minor_status,
 	 * sure we use the same mechanism switch as before.
 	 */
 	if (!ctx) {
-		if (mech_type == NULL)
-			mech_type = GSS_KRB5_MECHANISM;
-
 		ctx = malloc(sizeof(struct _gss_context));
 		if (!ctx) {
 			*minor_status = ENOMEM;