improved ZSTD_createCCtxPool() cancellation

improved ZSTD_createCCtxPool() cancellation

use ZSTD_freeCCtxPool() to release the partially created pool.
avoids to duplicate logic.

Also : identified a new difficult corner case :
when freeing the Pool, all CCtx should be previously released back to the pool.
Otherwise, it means some CCtx are still in use.
There is currently no clear policy on what to do in such a case.
Note : it's supposed to never happen.
Since pool creation/usage is static, it has no external user,
which limits risks.