Fix call to pcap_dispatch

This fix allows using zmap with libpcap 1.5.1, the semantic difference
of values 0 and -1 of this parameter is not entirely clear:

    "(In older versions of libpcap, the behavior when cnt was 0 was
    undefined; different platforms and devices behaved differently, so
    code that must work with older versions of libpcap should use -1,
    not 0, as the value of cnt.)"

http://www.tcpdump.org/manpages/pcap_loop.3pcap.html

Just using -1 worked for me.