COVER_selectDict() uses stale customDictContentEnd after ZDICT_finalizeDictionary modifies dictContentSize

**Describe the bug**
In `lib/dictBuilder/cover.c`, `COVER_selectDict()` computes `customDictContentEnd` at line 1055 based on the initial `dictContentSize`,Then at line 1069, `ZDICT_finalizeDictionary()` is called and its **return value** overwrites `dictContentSize`,ZDICT_finalizeDictionary()` returns the actual dictionary size after adding the header, which can differ from the input `dictContentSize`. However, `customDictContentEnd` is **never updated** to reflect the new `dictContentSize`.

**To Reproduce**
The currently public API cannot easily trigger this bug, but the bug does exist in the code logic.

**Expected behavior**
Update `customDictContentEnd` after `ZDICT_finalizeDictionary()` modifies `dictContentSize`.