Fix: ctf: notif-iter: do not call request_medium_bytes() when not needed

Fix: ctf: notif-iter: do not call request_medium_bytes() when not needed

Do not systematically call request_medium_bytes(), that is, at the
beginning of read_dscope_begin_state(), because the scope field which is
about to be decoded might be empty, an already aligned structure
field or the equivalent (through a variant field for example, of which
the tag is outside this scope).

To do this we also need to make bt_btr_start() accept a size of 0. In
this case, if the field to decode is an empty structure which is already
aligned, then bt_btr_start() does not need to consume any bit and
returns BT_BTR_STATUS_OK with 0 consumed bits. In this case, the next
notif-iter state is not a continuing state, thus request_medium_bytes()
is not called yet.

Checking for a bad state when request_medium_bytes() receives
BT_NOTIF_ITER_MEDIUM_STATUS_EOF from the medium is easier. Good states
are:

* If the current packet size is set:
** Current packet offset is current packet size.
* Otherwise:
** Current packet offset is 0.
** Last event header offset is set and current offset is equal to it.

All other states are invalid.

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