Fix: lib: don't assume that streams have packets in auto seek

Fix: lib: don't assume that streams have packets in auto seek

We get a segmentation fault when trying to instantiate a trimmer
downstream of a component that creates streams without packets.  The
reason is that the line changed by this patch assumes that events are
always within packets.  However, it is possible (since
26fc5aedf "lib: make packets and packet messages optional, disabled by
default") for stream to not use packets.  In that situation, the
`packet` field of `event_msg->event` will be NULL.

Fix it by using `event_msg->event->stream`, which is expected to be the
same thing as `event_msg->event->packet->stream`, in the case where the
stream uses packets.

A test exercising this is added by a following patch in this series.

The stack at the point of the crash is the following:

    #0 0x7f2235db23ab in auto_seek_handle_message /home/smarchi/src/babeltrace/src/lib/graph/iterator.c:1419
    #1 0x7f2235db330a in find_message_ge_ns_from_origin /home/smarchi/src/babeltrace/src/lib/graph/iterator.c:1567
    #2 0x7f2235db4b1a in bt_message_iterator_seek_ns_from_origin /home/smarchi/src/babeltrace/src/lib/graph/iterator.c:1790
    #3 0x7f2230abf6a4 in state_seek_initially /home/smarchi/src/babeltrace/src/plugins/utils/trimmer/trimmer.c:1095
    #4 0x7f2230ac2f3b in trimmer_msg_iter_next /home/smarchi/src/babeltrace/src/plugins/utils/trimmer/trimmer.c:1920
    #5 0x7f2235dae530 in call_iterator_next_method /home/smarchi/src/babeltrace/src/lib/graph/iterator.c:808
    #6 0x7f2235daefc6 in bt_message_iterator_next /home/smarchi/src/babeltrace/src/lib/graph/iterator.c:855
    #7 0x7f2230d1b458 in details_consume /home/smarchi/src/babeltrace/src/plugins/text/details/details.c:476
    #8 0x7f2235da3d61 in consume_graph_sink /home/smarchi/src/babeltrace/src/lib/graph/graph.c:456
    #9 0x7f2235da40e0 in consume_sink_node /home/smarchi/src/babeltrace/src/lib/graph/graph.c:498
    #10 0x7f2235da4875 in consume_no_check /home/smarchi/src/babeltrace/src/lib/graph/graph.c:572
    #11 0x7f2235da510b in bt_graph_run /home/smarchi/src/babeltrace/src/lib/graph/graph.c:636
    #12 0x563a71c8127a in cmd_run /home/smarchi/src/babeltrace/src/cli/babeltrace2.c:2503
    #13 0x563a71c8218a in main /home/smarchi/src/babeltrace/src/cli/babeltrace2.c:2691
    #14 0x7f22351f2b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
    #15 0x563a71c73709 in _start (/home/smarchi/build/babeltrace/src/cli/.libs/babeltrace2+0x1f709)

Change-Id: Ic7ed3927d5ad1ca04833248a97723f0d8c4e4907
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3104
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>