Fix: Missing description NULL check in bt_ctf_clock when serializing

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Fix: Ensure a packet context is a structure

bt_ctf_stream_class_set_packet_context_type was checking the current
stream packet context's type instead of the one being passed.

Added a test to validate that the check is done appropriately.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add stream_event_context accessors

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Fix: Only allow setting a packet context on a non-frozen stream class

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Cleanup: Missing whitespace in comment

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>