lib/chkhash.c: is_valid_hash(): Accept '*' as the hash

lib/chkhash.c: is_valid_hash(): Accept '*' as the hash

This is widely accepted as an invalid hash, to remove password access
for an account (that is, no passwords will match the "hash").

Fixes: c44f1e096a19 (2025-07-20; "chpasswd: Check hash before write when using -e")
Closes: <https://github.com/shadow-maint/shadow/issues/1483>
Closes: <https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1124835>
Reported-by: Chris Hofstaedtler <zeha@debian.org>
Reviewed-by: Chris Hofstaedtler <zeha@debian.org>
Cc: vinz <mmpx09@protonmail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>