Fix fuzzing with ZSTD_MULTITHREAD

[danlark1]

At Google we fuzz zstd without ZSTD_MULTITHREAD but we want inputs to be as much as reproducible. It allows us to test new fuzzing methods for our fuzz team internally and have more horsepower to find bugs

[terrelln]

Looks good to me, thanks @danlark1!

[danlark1]

Don't merge for now, please :)

    case ZSTD_c_nbWorkers :
#ifndef ZSTD_MULTITHREAD
        // Must be zero.
        RETURN_ERROR_IF(value!=0, parameter_unsupported, "not compiled with multithreading");
        return 0;
#else
        FORWARD_IF_ERROR(ZSTD_cParam_clampBounds(param, &value), "");
        CCtxParams->nbWorkers = value;
        return CCtxParams->nbWorkers;
#endif

UPD now should be good, thanks