When san_bump_grow_locked fails, it sets sba->curr_reg to NULL.
The old curr_reg (saved in to_destroy) was never freed or restored,
leaking the virtual memory extent. Restore sba->curr_reg from
to_destroy on failure so the old region remains usable.
While initially this file contained helper functions for one particular
test, now its usage spread across different test files. Purpose has
shifted towards a collection of handy arena ctl wrappers.
