Freeze original field type copied on validation

Freeze original field type copied on validation

The following situation should fail:

    field_type = create a structure FT with a sequence
                 or variant FT field
    ec = bt_ctf_event_class_create("ec")
    bt_ctf_event_class_set_payload_type(ec, field_type)
    sc = bt_ctf_stream_class_create("sc")
    bt_ctf_stream_class_add_event_class(sc, ec)
    modify field_type in any way -> should fail

In this scenario, field_type gets copied when the
validation process is executed in bt_ctf_stream_class_create()
because it contains a sequence or variant FT.

The original field type, field_type, should also be frozen
when this validation succeeds so that it behaves like it was
not copied in the first place.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>