Add changelog for gh-9218

The bug was fixed in the small library:
 - slab: fix NULL ptr deref in assertion in slab_get
   https://github.com/tarantool/small/commit/ef77efacd452cb90caea2caf22d266f791c95ec3
 - slab: fix uint32_t overflow in slab_capacity
   https://github.com/tarantool/small/commit/77203600a7c645d97bce56f901eec25de0b29d6e

The small library submodule was updated in commit ebafd684
("small: bump version").

Closes #9218

NO_DOC=changelog
NO_TEST=changelog

(cherry picked from commit 1fc6b4a1)