Fix: `ctf` plugin: msg-iter.c: do not switch packet twice

Fix: `ctf` plugin: msg-iter.c: do not switch packet twice

Issue
=====
In read_packet_header_begin_state(), bt_msg_iter_switch_packet() is
called systematically at the beginning of the state. However,
buf_ensure_available_bits() can fail below (the medium can return
`BT_MSG_ITER_MEDIUM_STATUS_AGAIN`), and read_packet_header_begin_state()
will be called again next time, calling bt_msg_iter_switch_packet()
twice for the same packet.

One of the problems of calling bt_msg_iter_switch_packet() twice is
that, the first time, the current snapshots are copied to the previous
snapshots and the current snapshots are reset; the second time, the
current (reset) snapshots are copied to the previous snapshots. This
shatters the whole snapshot states. Because the medium installed by
`src.ctf.lttng-live` can return `BT_MSG_ITER_MEDIUM_STATUS_AGAIN`, all
the messages that depend on the previous snapshots, like the discarded
events/packets messages, are wrong.

Solution
========
In `msg-iter.c`, add a new `STATE_SWITCH_PACKET` state which is
dedicated to doing what bt_msg_iter_switch_packet() used to do. Once it
completes, the next state is `STATE_DSCOPE_TRACE_PACKET_HEADER_BEGIN`.

read_packet_header_begin_state() does not switch packets itself now (the
`STATE_SWITCH_PACKET` state already occured at this point), so it can
safely occur twice.

Known drawbacks
===============
There's one more state to handle per packet, but this have a significant
performance hit.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reported-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Change-Id: I24dd2482762c61e339ab0310fddee6c9aa69d438
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1833
Tested-by: jenkins <jenkins@lttng.org>