zstdmt : correctly check for cctx and buffer allocation

zstdmt : correctly check for cctx and buffer allocation

Result from getBuffer and getCCtx could be NULL when allocation fails.
Now correctly checks : job creation stop and last job reports an allocation error.
releaseBuffer and releaseCCtx are now also compatible with NULL input.

Identified a new potential issue :
when early job fails, later jobs are not collected for resource retrieval.