ipif: eat_optionalstr: understand that *argp may be 0

[userv-utils] / ipif / ipif

# See /usr/share/doc/userv-ipif/service.c.txt
#
if ( grep calling-user-shell /etc/shells
   & glob service-user root
   )
        reset
        no-set-environment
        no-suppress-args
        no-disconnect-hup
        execute /usr/lib/userv/ipif /etc/userv/ipif-networks --
fi