lib: add missing MIP checks in bit array flag functions

lib: add missing MIP checks in bit array flag functions

Add some MIP checks, it seems like an oversight that these functions
don't have them.

I think that functions `bt_field_class_bit_array_flag_get_label()` and
`bt_field_class_bit_array_flag_borrow_index_ranges_const()` don't need
such checks, because it's not possible to get hold of a
`bt_field_class_bit_array_flag *` without going through function that
has the appropriate MIP check before.

Change-Id: I0848f414772aa7bf6c252d8e81ed60b20ccce14a
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/13397
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>