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.

lib/compress/zstd_compress.c

@@ -2627,9 +2627,9 @@ size_t ZSTD_compressBegin_usingDict(ZSTD_CCtx* cctx, const void* dict, size_t di
 }
 
 
-size_t ZSTD_compressBegin(ZSTD_CCtx* zc, int compressionLevel)
+size_t ZSTD_compressBegin(ZSTD_CCtx* cctx, int compressionLevel)
 {
-    return ZSTD_compressBegin_usingDict(zc, NULL, 0, compressionLevel);
+    return ZSTD_compressBegin_usingDict(cctx, NULL, 0, compressionLevel);
 }