A changelog for buffer overflow in decimal_unpack

Add a forgotten changelog entry.

Follow-up https://github.com/tarantool/security/issues/17

NO_DOC=changelog
NO_TEST=changelog

(cherry picked from commit 2be81637)

changelogs/unreleased/ghs-17-fix-decimal-unpack-overflow.md

+## bugfix/core
+
+* Fixed a possible buffer overflow in `mp_decode_decimal()` and
+  `decimal_unpack()` when an input string is too long (ghs-17).