lib/alg-yescrypt-platform.c: Fix -Werror=sign-conversion.
lib/alg-yescrypt-platform.c: Fix -Werror=sign-conversion.

In 894aee75433b4dc8d9724b126da6e79fa5f6814b we introduced some
changes, which show is this error when building with GCC v12.2.1.

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