Medium: ldirector: fix using service name for real servers (bnc#836759)

virtual = mail:smtp
        checktimeout = 30
        checkinterval = 120
        failurecount = 1
        protocol = tcp
        scheduler = nq
        real = g1:smtp masq

yields:

Use of uninitialized value $protocol in getservbyname at /usr/sbin/ldirectord
line 5115, <CFGFILE> line 22.

Only the "real" setting is affected.