Fix: bt2: incref Py_None in get_msg_range_common on error

Fix: bt2: incref Py_None in get_msg_range_common on error

When get_msg_range_common processes a result that is not OK, it
returns a tuple whose second element is None:

  (status, None)

However, when setting the second element of the tuple to None, we are
missing an incref of Py_None.  This incref is necessary, because
PyTuple_SET_ITEM steals the reference of the object we pass.

This patch fixes that.

Change-Id: I8a5af2853172a8399dd1779d90e0fcb8fc265032
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1731
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>