chpasswd: Check hash before write when using -e

chpasswd: Check hash before write when using -e

Add is_valid_hash to prevent adding a bad hash in /etc/shadow (and so prevent user to be lock) when using chpasswd -e

    # before
    echo 'vinz:test123' | chpasswd -e
    grep vinz /etc/shadow
    vinz:test123:20280:0:99999:7:::

    # now
    echo 'vinz:test123' | sudo ./chpasswd -e
    chpasswd: (line 1, user vinz) invalid password hash
    chpasswd: error detected, changes ignored