Fix error in win32 get_ipaddr4 causing buf overrun

In libnet_get_ipaddr4 (__WIN32__ defined) the
PacketGetNetInfoEx(l->device, &ipbuff, &npflen) takes a pointer to the
count of buffers in the last parameter, not to the size of one buffer.
This causes buffer overruns (tested in MSVC 2013 ultimate), changing the
npflen to 1 solves this issue.