Fix crash on systems with tuntap interfaces

In voglutils.cpp: getifaddrs() returns a linked list
of network interfaces of type 'struct ifAddrStruct'.
This may include tuntap interfaces of which the ifa->addr is NULL
To replicate this behaviour one can add a tuntap interface like this:
$ sudo ip tuntap add tun99 mode tun
To remove the previously created tuntap interface:
$ sudo ip tuntap del tun99 mode tun
These tuntap devices may be created by VPN software such as
OpenVPN.

To work around this, we simply check whether
ifa->addr is not NULL before proceeding.
Merge pull request #11 from gotschelm/fix_get_ip_addr

Fix crash on systems with tuntap interfaces