Update libnet_resolve.c

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.