Fix: maybe-uninitialized warning in bt_ctf_stream_flush()

Fix: maybe-uninitialized warning in bt_ctf_stream_flush()

GCC warning:
  In file included from ../../include/babeltrace/ctf-writer/fields-internal.h:41:0,
                   from ../../include/babeltrace/ctf-writer/field-wrapper-internal.h:26,
                   from ../../include/babeltrace/ctf-writer/event-internal.h:37,
                   from stream.c:32:
  stream.c: In function ‘bt_ctf_stream_flush’:
  ../../include/babeltrace/ctfser-internal.h:568:36: error:
  ‘packet_context_offset_bits’ may be used uninitialized in this function
  [-Werror=maybe-uninitialized]
    ctfser->offset_in_cur_packet_bits = offset_bits;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~
  stream.c:1608:11: note: ‘packet_context_offset_bits’ was declared here
    uint64_t packet_context_offset_bits;

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ia7457b1a444290ba1a71459bfaf707812227899c
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1029
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>