zstdmt: fixed minor race condition

no real consequence, but pollute tsan tests :

job->dstBuff is being modified inside worker,
while main thread might read it accidentally
because it copies whole job.
But since it doesn't used dstBuff, there is no real consequence.

Other potential solution : only copy useful data, instead of whole job

lib/compress/zstdmt_compress.c

@@ -344,7 +344,9 @@ void ZSTDMT_compressChunk(void* jobDescription)
             job->cSize = ERROR(memory_allocation);
             goto _endJob;
         }
+        ZSTD_PTHREAD_MUTEX_LOCK(job->jobCompleted_mutex);   /* note : it's a mtctx mutex */
         job->dstBuff = dstBuff;
+        ZSTD_pthread_mutex_unlock(job->jobCompleted_mutex);
     }
 
     /* init */