If the --user argument user[.group] contains a dot, always bail out if no group has...

If the --user argument user[.group] contains a dot, always bail out if no group has been specified.

Previously the intended, but undocumented, behaviour was to
try interpreting the whole argument as user name, but the
detection was flawed and checked for '0' isntead of '\0',
thus merely preventing group names beginning with a zero.

Remove the incorrect check instead of fixing it to get
the documented behaviour.