cpp-common/bt2: check return value in `CommonVariantField::selectedOption()`

cpp-common/bt2: check return value in `CommonVariantField::selectedOption()`

Instead of ignoring the return value of
bt_field_variant_select_option_by_index(), check it with
BT_ASSERT_DBG().  This probably won't ever make a difference since
bt_field_variant_select_option_by_index() can't fail for now, but it
looks better.

Change-Id: I2354adfa1358ef0bd77b1d7c6604eef8ccaaa018
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11950
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>