Fix the (not happening) explicit null dereference found by CovScan.
Fix the (not happening) explicit null dereference found by CovScan.

This small change fixes the false positive finding by CovScan in
the blockmix functions from alg-yescrypt-opt.c.

besser82/libxcrypt,besser82/libxcrypt,besser82/libxcrypt,besser82/libxcrypt