The glibc crypt() implementation in RHEL 8 apparently sometimes
performs misguided "validation" of the salt even for "$6$" and
fails with EINVAL instead of actually validating the password.

Throwing an error on validation failure is not what existing
code expects, so just return a failure instead in this case.

Fixes #10050.

See also https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=714219